OBS Ford fuel quantity discrepancy

I recently purchased a 1997 Ford F-250 Heavy Duty with the 7.3L Powerstroke diesel and 212k miles on it. Two fuel tanks (front and rear) with a two-way selector on the dash. When I drove it off the lot, both tanks showed near E so my first stop was the nearest gas station.

I put 18.3 gallons in the front tank before the pump clicked off, which seemed about right because the owner’s manual specs the front tank at 19 gallons. Then 16.7 into the rear tank before I ran out of fuel discount at 35 total gallons.

Started her up, and the reading on the front tank was about 1/4 full. Okay, maybe something wrong there (sending unit, electrical fault, etc.) but nothing to be too worried about. The family truck growing up did the same thing. Switched over to the rear tank and the needle slowly went up to F, which seemed reasonable as the stock rear tank is specced at 18.2 gallons. Drove home and was able to switch between tanks without a problem.

Today I decided to take a quick look underneath just to see what I might be dealing with and to my surprise the rear tank is plated for a 45-gallon capacity:

Clearly an undocumented option or an aftermarket swap that was completed when the truck was new. Explains why there’s no room for a spare tire where it should usually go.

Bottom line: I filled a 19-gallon tank completely and the gauge reads 1/4 full, and filled a 45-gallon tank less than 40% and the gauge reads full. What might be going on here?