Regardless where the play stops, where a penalty puts it. It is always on the hash mark. I have been trying to see if they are moving it forwards or backwards to get it to the as mark.
Yes, I've noticed this too.
Not sure why refs do this. Maybe it's easier to mark off penalties when the ball is on the hash?
Also, it permits refs to more easily declare that a first down has been made. For example, if, after a play, the ball is marked on the hash that is exactly 10 yards from where it was on first down, then there's no need to measure for the first down.
I think the refs start marking the ball more precisely (e.g., between hashes) when the offense is near the goal line (inside the 10-yard line).