OK, I'll diverge and disagree with everybody. I don't think it's your nocking point (string loop) nor is it string creep. You're shooting high one day and low the next, then high another day. Strings and nocking points just don't move around like this. If they did the POI would change, but in one direction. It would only go in one direction, not up one day and down the next.

I would suggest that it is your shooting form. Maybe you're just anchoring slightly higher or lower on any given day. That's my opinion and I'm sticking by it.

Do yourself a favor the next time you change strings and do not use Zebra's. Get hold of Gibblet and he'll make you a set better than Zebra and at about the same cost or maybe even a touch less.