I might be missing something, but 0% vert and horizontal should give the same values and also regardless of aspect ratio/dimentions but

1. they're not the same

2. 0% horizontal scales with resolution?

The first two is due to rounding. The calculator rounds to the nearest 4 decimals, so you get 100.8912 with the first one and 1008.9175 with the second one.  The first one is rounded from 100.89175xxxx.

Regarding 0% on horizontal vs. vertical, they are only the same if the aspect ratio is the same. If you change the aspect ratio they will be different since the FOV changes. They will each change if the FOV changes in the plane their name indicates.