According to the site, the order of difficulty is

Simple
Very Easy
Easy
Average
Tough
Difficult
Very Difficult
Impossible

but I have a quiz rated "Very Hard." Are Hard and Difficult interchangeable terms? If so I'm supposing that FT just switched terms midway and some stayed the old way. If not, I don't get where "Hard" fits in.

Interesting, maybe you're just special.

I would like to say that what I do as an editor, is to see if the difficulty rating is high for a good reason. If a quiz is rated very hard or difficult or even tough for the wrong reasons, or it needs some structural work, then some people tend to shy away from it.
A wrong reason is an incorrect spelling or variant spelling on an FITB, or a misleading question. FITB's generate the most errors in general, so you need to check if it's for the right reasons.
I view FITB's as the rare color on my palette when I make a quiz, except for my word games and those, well, that's the whole thing, they require close attention to possessives.
So when you craft a quiz, you want to ask yourself, if someone who knows a bit about the subject would get a decent score and or learn something about the subject.

Sometimes the subject itself is what makes the difficulty rating go up, but that doesn't stop us from doing it.

I think that I, like many of us who make quizzes, enjoy making them in all ranges and for different purposes.
I don't let the lack of an audience bother me unduly...nor the ratings.