If you read up on the scientific models of memory, you'll see that it's divided in many ways and areas. You may be familiar with the idea of short-term vs long-term memory. That's elementary compared to the more complex models that scientists have come up with, with more divisions and specialization. Each area has its strengths and weaknesses in terms of capacity and recall. The wikipedia page is a nice overview: Memory - Wikipedia. When we are asleep, it's likely that some memory areas are more suppressed than others, just like reasoning and sensation. Lucid dreaming seems to wake up some areas of the brain that usually aren't active during normal sleep but it's not always the same as waking brain function, so memory function is impaired in interesting and unusual ways.