"Creep"
Okay. That's exactly what I've been experiencing with my smooth target grips, and it's the reason I'm reconsidering finger grooves. I used to have wood Hogues that didn't seem to have that problem, but mine were SO tight that they were practically impossible to get on and off, so I got rid of them. I've read posts from guys who are perfectly happy with their wood Hogues, so I understand that mine were probably a fluke, but I've just decided to stick with traditional two-piece grips in the future.
Edit to add: By the way, I think the term "creep" IS commonly used in gun circles, but I'm pretty sure it refers to the trigger.