I've noticed that I grip each pistol a bit differently, and I didn't think much of it until I put a Hogue grip on my Hi-power .40. When I'd shoot my Hi-power nine right after that, I'd shoot to the right. Put Hogues on the nine, problem disappeared. Just a theory, but I suspect by the time you get back to it, you're using a slightly different grip. Easy enuf to test. Put it down after the first group, get a drink, come back and shoot it again (without firing another pistol first).