2003 S-10 belt replacement too short

I bought a serpentine belt for my 2003 S-10 ZR-2 today. The part number was K060950 or 950K6 at O'Reilly. When I tried to put it on, it was too short.
I could not get it under the fixed idler pulley and if I had, it would have rubbed against the mount arm for the tensioner.
This is the second belt I have bought for the truck. Back at 30k miles I replaced the factory belt and had the same problem. I wound up getting a belt with number 6PK2420 or 952K6 which has worked well for 50k miles.
I still have the original factory belt which I carry as a spare. It has a number 6PK2415. Just to verify, I put it back on the truck. It is tight but I had no trouble getting it on.
O'Reilly traded me for a belt number K060956 or 955K6, ostensibly the next longer length they have. When I put it on the truck, it worked perfectly. It is a bit longer than the factory belt, but not much.
Does O'Reilly have the wrong part number in their system? Did Chevy put a different belt on the ZR-2 than the regular S-10?
Bill Gee

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