Loops drifting same song, same loop manually beatmatched

Hi there, 

I first noticed when playing a set that I would have to correct a beatmatched loop more than I thought I should.

So I did a little test. Two songs at exactly the same BPM never drifted apart for their entire duration. However, the moment that they were looped, they could barely make it past about 4 bars before the beats were galloping. This is bizarre because they were at exactly the same BPM and didn't drift when not in loop.

Okay I thought, maybe the songs weren't properly analysed so I take the same song, at the exactly the same time in the song and put a 4 beat loop on both and yet again the same 'deck' was starting to move ahead of the other after about 4 bars. 

Quantize mode is on. So what can be the explanation for this?