Hi metropoly,
That is strange. It may be due to USB jitter (a USB Hub for instance, bad quality USB cable, or your computer, ...) or a very high tempo. Tempo > 300BPM is not supported by DrumBrute.
However, if it sounds synced, you should not worry about the display. The display is just informative (competitors usually do not display the BPM while synced, I guess because of jitter issues). Look at Ableton Live tempo giggling when it is slaved.
Regards,
Y.