Your anchor is a little bit different and/or bow torque. As you well know, you just don't have to move your anchor point much at all to move your impact point. Your case is rather interesting in that your groups move, so you're consistent each day, but not consistent from one day to the next.

Now having said that, I would still triple check and make sure everything is tight, the bow is still in tune, and also make sure your arrows aren't damaged, ie cracked.

I'm betting it's an achor thing though. And a subtle one at that.