More like it is totally off. With a sniper rifle, aimed shot showing about 65% probability to hit in reality has about 5% chance of hitting. Checked that by using multiple restarts on a mining/Asia UFO crash map, because there it repeatedly deployed one sniper where I could crouch and fire at an alien deployed in the same spot on turn one in 19 out of 20 loadups. And even when hitting, a kill (even against a non-armored one) was scored in about 1/10 cases.
Now, the funner part. A rocket launcher shot at the same target from same spot with displayed hit probability of about 30% was scoring a direct hit in two out of three shots (and in all other maps, it's apparently the most accurate weapon). A kill was scored only in half the hits, though. And frankly, it seems to cause almost no splash damage whatsoever. I only twice managed to kill someone with splash damage, and it includes the cases with aliens standing adjacent to each other in a small (3x4) room, so there just was NO WAY to not catch them in the explosion. So I score a direct hit on one, it dies, but the other just goes like, "Huh? Someone farted?"
What's the problem with using the same thing to determine hit probability when aiming and when the actual shot is fired?