Funny. I noticed this too, when snowboarding yesterday. I use a very good sintered base which I maintain very well, so that can't be the problem. A friend of mine, who has a extruded base which he doesn't maintain that well, especially yesterday, has the same, he has it worse than me. But we have it both on the same box, so we think that it's because of the box + be sure your base is smooth as hell!