Reply to Two DJs, Traktor + Virtual DJ, Tempo Inconsistency "WTF Moment"

Okay, so for the last few weeks I've been playing a new venue with a mate of mine, and doing a sort of "you play a few tracks, then I'll play a few tracks, and so on" kinda deal. We've both got our own setups, me using Traktor and him using Virtual DJ, and I've noticed something REALLY weird going on... basically, if he's playing a track at 130bpm, and I cue up a track at 130bpm, I can start mine off perfectly in sync, and then it drifts, almost as if his is playing at 129 or 131...

Now this is what I don't get; correct me if I'm wrong, but 130bpm means quite literally, 130 beats per minute. Now unless Traktor or Virtual DJ has a different definition of a "minute" than the rest of the world, then what the hell is going on?

Obviously the ideal solution would be for him to switch to Traktor, but I have a feeling it'll take a while for him to come around to the idea lol. In the mean time, am I just mental, or is there really something funny going on here?