When the skins punted in OT hoqw many people said this?

[htownskinfan]

Since this thread is title is in reference to punting,I have a question I want to see if anyone can answer and i didnt want to start a new thread about it.
How come when someone punts the ball{offense} when its touched by anyone on defense{lets say about the 20 yard line}and it continues to roll and is downed on the 5 yard line they bring it back to the spot it was first touched{20 yard line}.But if it is touched by defense{lets say 10 yard line}and it rolls into the endzone they call it a touchback instead of putting it on the 10 yard line? Ive never understood that