Don't feel bad, several times I've replaced those deck belts and scratched my old head saying I know I have the right belt, how come it don't fit.
Then Duh, routing is wong er wrong?
FWIW:
order a Goodyear
instapower series belt or OEM cub, they are made to run on those backside idlers and 1/4 twists of a mule drive when necessary.
Evilpay are usually the best prices.
Also take the idler off it's pivot and grease it well, as they hang up and ruin a new belt quick.
Also inspect your idler spring for hook attachment breakage.