I have a copy of an early Willys manual, and it shows the 161 L-6 at 75 hp and 6.9 compression, while the F-6 was 90 hp and 7.6 compression. Maybe the Kaiser L-head was a little higher compression explaining the 80 hp rating in a Kaiser? I've got one document saying the Kaiser version was 7.0 compression, but that is a very small change and would not account for 5 hp. More likely they simply put a not so conservative number on it. I can say you could not get 15 hp from compression, that's a big percentage.
The F-4 did have different compression ratios, 6.9, 7.4, and 7.8, with powers of 72 or 75.