To remember something is to repeat the same brain signals that once were transported through the neurones when the incident first occurred. That is why a memory is like a journey back in time. However, since the details might be blurry, and some things are lost, it is not a total recall...the "time travel" is not perfect. The imperfection must be due to changes in you as well as your surroundings, so that the memory signals becomes too distorted to be the very exact. If this distortion did not have happen I am sure that we could have seen our own childhood like the eyes were a film camera.
I think that memory signals can explain lack of imagination as well as a too vivid imagination and I will try to "solve/analyze" this from a philosophical point of view:

 

I think that memory signals could be a key to how the brain works, some people's brain is working well, signals are repeating themselves more accurately and naturally those people also have a better recollection than others. Some things, for example smells, seem to trigger recollections. The cause of that might be that it is connected to survival (smelling food/poison/animals and so on). We also keep certain memories better simply because they remind us about something that was special for us in childhood and therefore is more fitting to the pattern of our brain. We tend to remember the good or bad, while more bland impressions disappear. Is imagination a product of reconstructed memories? If you use your imagination to create a vision, you might use your memory signals to do so. Maybe you, when you try to envision things, HAVE to create it from moving around already existing memory signals to create a new "melody" (fantasy) from them. That might be the reason why so many children nowadays have such bad imaginations, because they don't interact either with their family, or by reading books...and when they are with friends they all stare on a screen. This does not make a good foundation of experiences to build from. It seems logical that one cannot create something out of nothing. No track without rails. The ability to compose "new memory based imagination music" will differ between people, leaving some more equipped in doing so, like for example writers. When it comes to disabilities, or maybe disturbances is a better word for it, I thought about hallucinations, when people hear och see something that is not there. Obviously this -something- is created in their own brain. Maybe their deficiency is that they subconsciously create a tune, and after doing so they can't turn it off, Instead their created imagination keeps playing back, like a scratched record. And just maybe this melody is more "perfect" than other distorted memory so that the hallucinating people can't differentiate between a created tune (imagination) and an experienced one (reality). To have an OCD (Obsessive Compulsive Disorder) might also be this kind of repetitive "hiccup". My conclusion is that the human brain can have trouble differentiating between memories and its own creations. Normally a healthy brain can probably tell them apart by the way a real memory and what is actually happening is vividly perfect...while the distortion, bluriness and loss of details signify a memory as being "less perfect". How else would we know what is real and is actually happening and what is not?