Fred,
Could have something to do with the paper work when you order the car. My 69 Cobra had 3:91s on the order form and it is in the 69 Cobra registry, the gear ratios are given as 3:91 posi-trac. I have never taken the ring gear and pinion out and counted the teeth to see if they are 3:91s or 3:90s. However I do know that once when I was talking about ordering a spare rear end they only had the 3:91s and 4:11s i for that car.
Ron
