@michel > Kevin started a new thread here: http://forums.pioneerdj.com/entries/20681206-djm900-bpm-readout-does-not-work-as-advertised
I messaged Pulse on this matter and I agree with him that the DJM-900 follows the BPM as accurately as it can in the video that Kevin provided. The DJM-900 does not receive the entire waveform of tracks and thus cannot predict a 100% accurate BMP all the time. It uses what information is being provided from the CDJ and tries to calculate the BPM based on how the song is reacting. Since the song provided in the video doesn't have much for the DJM-900 to pick up and calculate, it does it's best with what frequencies are being provided.
Hopefully that thread will explain things a bit better.