Dxtory : Game audio and microphone audio not in sync

[Xirix]

Just recorded some game footage with commentary with Dxtory, having the microphone as the second channel. Just checked both videos, one is 2 hours one is just 30 minutes, they have the same problem... the commentary audio is about 30 seconds to 1 minute late roughly, previous tests I've done it synced up perfectly.

The only thing I can think of, I accidently forgot to do a benchmark test before recording, could this be the cause? I can probably easily re-sync the audios in vegas, but I'd like to know why this kind of this happened so I can avoid it in the future.