Is there any history behind why one shifted to American while the other remained Latin? Or is it just one of those language quirks to mess with our heads?
Because politicians are goobers.
Heh. My original had a reference to rednecks (the Virginia thread in the Elections forum gave rise to the question), but the diversion was longer than the question. Looks like I took out ‘Real’ but left ‘American’.
The free online dictionary says gubernator is the Latin word for governor. So maybe the switch between Latin and English is to not offend the elected gubernors. 
I’m 99% certain the “Gubernator” was a weapon invented by Dr. Doofenschmertz on “Phineas and Ferb”. Which turned the entire tri-state area into a peanut farm or something.
I’ve always called the governor the goober. Here in Oregon, I get double the fun as I can refer to Governor Kitzhaber as “Goober Cat Slobber”. Nyuknyuknyuk.
Goobernatorial is one of my favorite funny sounding words.
It’s a general feature of the English language. There are a lot of cases where Germanic nouns are coupled with adjectives from Latin or Greek. Like you have the “sun” up in the sky, but solar power, solar radiation, and solar eclipses. American astronauts went to the “moon”, but drove around on the moon using a “lunar” rover. If you are having “lung” problems, you can see a “pulmonary” specialist. If you are looking for “whales”, you might take a hint from Captain Kirk and try the “Cetacean” Institute. If you don’t get along with your “brother” your are having “fraternal” issues. Numbers you can calculate or express using your “fingers” are “digital” (yes, literally this is what digital logic is supposed to refer to - it implies an analogy with being able to work things out on your fingers).
Because anglicizing the adjective form never caught on. What would you say, “Governatorial”? That sounds like it should be specific to the Governator. 
Edit: Thanks for the extra background, robert_columbia!
For the same reason it’s a congressional race, not a congressal race. Euphony is a strong factor in etymology, and often a Latin form is used if it sounds better.
Glad I could help.
Other examples:
The herald of “spring” is the “vernal” equinox.
People go to “work”, but belong to a “labor” union.
One common issue with “teeth” is “dental” cavities.
When you punish someone’s “body”, that is “corporal” punishment.
If your abilities are on the “edge” of what is desired, you are a “marginal” performer.
A “king” has a “royal” palace.
If you work with your “hands”, you are doing “manual” work.
You can play games in the “water”, but belong to an “aquatic” sports league.
“Gubernatorial” was especially appropriate when Jimmy Carter, a goober farmer, became governor of Georgia. 
The EU is concerned (some might say obsessed) with standardisation across the various member countries. This is made more difficult by the language differences.
Meat is a good example and there are many many regulations about it. One of the basic problems is what sort of meat it is. Mutton, sheep meat, lamb are all the same thing and that’s just the English words. Add in mouton, schaap, montone and caoireoil, to mention but a few, and you can see the problem.
The solution was to revert to the latin root. So sheep meat is ovine, pork is porcine and beef is bovine.
Sounds like a Game Room thread! Come up with the…Germanic Noun/Latin Adjective Combo. Okay, the title needs work…
“Governor” is not a Germanic noun. It’s an anglicized Latin noun, via French.
Not sure if this is the case here, but sometime the same Latinate root finds it’s way into English multiple times, but by different paths. A word might find it’s way into English via French (or sometimes Spanish or Italian), and then another form will be borrowed directly from Latin at a much later date.
We also see this with genuine Anglo Saxon words that have cognates from Old Norse that made their way into English during period of Viking invasions (eg, shirt and skirt).
My grandmother was a minor politician in Alaska who knew then-Governor Ernest Gruening, whom she referred to affectionately as “the Gub.” Think of the fun if that had caught on as local or regional slang? All of this relates to the flexibility and creativity inherent and habitual in the English language, which began as a streamlined common tongue for interlopers in Britain who were linguistic cousins.
For comparison: My Russian instructor in college summed up a convoluted explanation of some Russian term with the remark: “The Russians do things for reasons known only to the Russians.”
And actually, before it was Latin it was Greek–the Latin gubernator is from the Greek κυβερνήτης or kybernetes, meaning “pilot” (in a nautical sense) or “steersman”.
In light of Arnold Schwarzenegger’s career, it’s interesting that κυβερνήτης /kybernetes also, by another route, gave rise to the word cybernetic and its various derivatives (like “cyborg”, short for “cybernetic organism”).
Yeah, well, nothing will ever top Das Gubernator.
This seems like a good bunch of folks to ask one of my long pondered questions: Why do the people known as Filipinos live in the Philippines? An F and a single P vs. a Ph and a double P?