Deck for John Deere Z710A

I have posted this in another thread.

Why does the belt on the tension arm ride at the very top of the double pulley. Nothing has changed on my deck since bought it except change belts,tension bearings and a idler pulley. I have had the mower for about 10yrs. The reason I have broke a few belts is that I cut along the road for about 1 1/2 miles picking up sticks. I just put one belt on and after about an hour the belt broke in to. But i have noticed that the tension army pulley is riding that the top which makes it pull slightly up from the blade pulley. I'm kinda stumped on what the problem is.

Because the pivot point on the tensioning arm is worn out of round.
This allows the pulley to sit slightly out of plane with the rest of the pulleys so the belt rides up
I weld the pivot holes then remachine the hole about 30 times a year on different decks.
On some decks where the tensioning arm is thick enough, they get a real bearing.