The CDJ is analysing the track as it reads it the waveform and beats into the memory (note: this is not buffering, just pre-reading to analyse the song). As it does that, the BPM may vary.
Yes, you CAN set hot cues on different tracks (and those DO buffer to memory!), this was one the of tricks performance DJs used with the CDJ-1000's to jump between tracks quickly (without using the track load buttons). You can store them all on one track and recall them, or you can store them to anywhere you want on the CD. Same goes for memory points.