Because the DM doesn't get roll to decieve to have the PC let him past and then adjudicate his own roll. I've never in my life seen skill rolls played that way, although I suppose that people do. It's the second bit, the let him past, that isn't a usual part of the player facing skill roll process. Players usually decide their own actions, not have them dictated by a skill roll. The only D&D exception I can think of is spells, but that's different.Why wouldn't you get the expected outcome? The only way I see that happening is that the players ignore the dice results when they're applied to their PCs.