I agree with Mikeaus. The reason that you can have normal memories is because your consious. So when your consious during a dream that would mean that instead of the dream be recorded as a subconsious memory and fractured or forgotten, it is be recorded in consious memory so it can be easily remembered in detail. But just like normal memories they can be forgotten if you drift from them, which happens when you awaken and that is when lucid dreams fracture and quality or parts of the dream are forgotten.