ring and pinion question

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I picked up a 1970 GMC Sierra Grande short bed(cut down). The PO decided to redrill the front and rear end to the small 5-4 3/4 lug pattern for aftermarket disc brakes and his wheels he had. I picked up a complete rear and and complete front end (hub to hub) for $100 from a buddy of mine. I measured both 12 bolts to make sure they were the same width. After yanking both axels out of both rear end I looked at the ring and pinions. Both are 3.07 ratios, the rear end under the truck is 14:43 tooth count and the spare rear end is 13:40 tooth count. Why would GM use 2 seperate ring and pinion setups for the same ratio?