Brian,
I suspect Arne's on the right track. I shoot 3U and found out quite by accident that I had imperceptibly moved my middle fingertip anchor from corner of my mouth to right against my incisor, just because it seemed more solid and repeatable. I'd guess this amounted to no more than 1/8th inch actual horizontal movement in my anchor, probably more like 1/16th. It had so little effect on POI with fletched shafts that I never noticed it and probably was compensating the sight picture within a few shots. The next time I bareshafted, which was probably months later, everything had "gone down the crapper" and it took forever for me to figure out what I'd changed. Also, don't know about the cold weather theory otherwise, but personally, my draw length will change noticeably from the dead of winter(greatly reduced practice, loss of muscle memory/back engagement) to summer (frequent practice/full back muscle expansion, but I'm aware of it and usually can counter with tip weight.