4.56 Rear Gears?!?!?!?!?!

[69C10@16]

I was playing around with a rpm-speed calculator this afternoon trying to figure out what the 700r4 would change my crusing rpm to. I typed in 3.73 rear gears with a 1:1 transmission ratio for final gear in the th350. I put in my tire size and all that and what it calculates is all wrong. So i started typing in other rear end gears because it is obvious that a PO was in the differential because there's blue gasket stuff all around the cover. I discovered that the closest rear gearing that matches what my truck actually does is 4.56.

Were these gears ever put out on these trucks or did someone change the gearing in my differential?

If anyone else wants to play around with their own calculator to see if I'm all wrong here's what I have.

Turbo-350
235/75/R15's.
At 55MPH Im running about 2800RPM give or take a few.