Ok.. why does it do that.. ?

When we experience an event in our waking lives,
there are points of reference, there is a layout to where the event took place

So if we dream of,  say ... walking in the park .. we see a park..
we see trees, we see all the things one would expect to find there,
and any actions we take, are actions we have taken before.
So the mind can present a clear, concise image.

Now, say we have been in the Astral and things happened there ?
Now what image does the mind use to represent this ? It has no referent to use,
there is no analogy to your waking life to call on

So the mind "clothes " the concepts in the closest thing it can find

For Example:

A common dream of mine.. I see Myself .. climbing a mountain,
There's a white Wolf with me, I have long white hair,
and silver eyes , ( which would be very cool .. but ) and there are misty clouds and strange 
looking trees, sometimes the climb is easy. Other times there are large rocks to be moved 
and fallen trees to climb over and so on and so on

Now this image and all the actions taking place in it, means

Times of growth and change on the path, it's the sign of working on adeptship
and learning new things, hence the mountain to climb the wolf is my guide,
the silver eyes are for seeing beyond the mundane. The dream is the minds attempt to clothe
the allegory of doing self improvement work on an astral level
Now, it must do this,  in a form that my waking mind, can understand
It's at a higher level than the normal world, 
so the mind has to come up with images that it can relate to

This "dream" always happens now when I am working on self improvement
and I have reinforced the imagery, but it took a long time to figure out
what it was and make the connection between recent events
and my own attempts at self improvement

Also , how "hard" the climb is and how many rocks are in the way,
give me pretty good indication of how well I am progressing on the chosen path,
and how much more work there is to be done

Now .. I am sure your thinking .. ok that's all well and good but
what has that to do with me.. ?
Two words

Guided Dreaming

Once you accept the concept that dreams are just the clothes for information we have gathered
and that such dream time is often astral travel, then the concept of doing it deliberately comes into play,
just like we do when we are in a meditative state and go astraling

This form is  much easier, because as I said, it's what we do while we are asleep anyway
So, how to make the dream time go in the direction we want it to go ?

Directions:

Like any trip, we need directions on how to get there
There are a variety of ways to do this. I use the Cabala myself
most of the time. I set in my mind before meditative sleep which "world" I wish to attend  and in my sleep
go there and learn what is to be learned, as each "world" of the Cabala has different things to teach,
this way is very effective, others include:

Making an Astral house and inviting others in to share knowledge, what we build on this plane is reflected above
.. as above so below ... most of the time, if we use the same space for ritual, we have built an Astral copy
of it just by sheer repetition And can " visit " that copy while on the Astral, at will.

For guided dream time, we  may need a more neutral space
a house, a meadow, or like me, a mountain, to represent why we are there and what we seek . 

No matter what referent we give the mind, the information gathered will be presented 
in as close a proximity to the image you give the concept, 
the more information we give the mind , the clearer the image will be

Ya  can't take take color pictures
with black and white film .... the Mind needs details...... :)

One of my first such places, I called the Grotto. Deep warm space ,with rock walls and lots of plants and water
.. a very comfy chair .. some trippy doors to come in by, and a very deep hole off to one side. 
That hole was my bottomless pit, that I would toss things into, that I no longer needed. 
Fears, angers, anything outdated and or incorrect in the mind ... 
Hook shot ... in it went !

I still visit that little nook from time to time, just to dust and add a thing or three
to the group, already off to oblivion :)

Like the image of the Asherkic records... most people see a library with stacks upon stacks of books,
or a hall of mirrors, where the past selves are displayed.
These are common images, simply because they work for many people

Now how does one Know if such information,
once we get it, is real ?

Clues:

How to tell fact from fiction ?  By what relates.
Much the same as when you are given information from within,
during meditation , you have to decide if the information gained
is valid, despite how the information is clothed

Figure out the imagery , ( there are a billion books on the subject of interpretation )
and interpret it literally, weigh that information against

What have you been doing..? 

What did you go out to seek ?

Does the information gained pass the tests of Reason and Logic ?

Make a list , write it all down, feelings ,interpretation of it's parts ,etc. And kick it around ,
open  yourself to concepts that are already in the brain, Meditate on it,
if it can pass all of those, it is likely real and you should act on the information
or otherwise pay heed

 Sounds like a lot of hassle right ?
It gets easier over time and with practice ... :)

Much like you would do for information the Mother hands you
in the day to day world on what to do, see " How to Know " what to do
As this is the same concept, the only difference is,
This information, you went looking for !

Don't worry about  missing something this way ... if it's important ..
trust me Mother will Make Sure, you get the point !
With an Astral Lighting bolt if need be :)

While a Great many dreams can be no more than twiddling your mental thumbs,
with practice, from time to time you can do real work,
and gain so much more than a few hours rest :)