Brains don't store everything in perfect detail. It only stores certain aspects of situations where importance is judged by the brain based on factors like how often it occurs, how big it is, etc. Therefore you have actually only stored a cloudy mush of information and whenever you want to recall something in more detail, your brain uses imagination (the creation of memory) to interpolate between its stored aspects. The more you repeat something, however, the more details are actually going to be stored, and the less imagination is needed to complete the picture.
Memories of real things are stored the same way as fantasies are. There are no difference between them apart from the source. What your brain does best is comparision, so it uses it to its full potential. To decide if a memory is real or a fantasy, it compares it to everything else it has stored. The more memories it matches with, the more real it is considered. Other than that, the brain has no way to decide what's real and what's not.
_________________
When superficiality reigns your reality, you are already lost in the sea of normality.