Yeah, penultimate is not really a superlative.
Literraly, it means "just before the last". "Ultimate" being "the last", and "pen" "just before".
It's funny that "ultimate" is used in a laudative fashion, while "last" is rather pejorative, unless used to means "last remaining". In this context, "penultimate" could mean "predecessor of the last remaining". A penultimate farmer could be, for example, a pregnant woman who happen to be the sole survivor of a planet-wide cataclysm. Her only children would be the ultimate farmer. Farmer, because being the sole survivor of a cataclysm don't remove the need to eat, and growing crops is the easiest way to get fed when you're alone in the world.