I think this is an example of a "purity spiral" that reduces the size of the audience by alienating outlying members.
It's the same thing that happened when I was an engineering student at university. The first test would come out and 30% of people would get C's or lower. They would drop the class. The next test would come out and a new cohort would get C's and drop, etc... The result was a distilled "pure" batch of engineers who employers like Microsoft could hire without worry.
That's not what we are going for on JetPunk. We are trying to grow the audience, not shrink it.
A few interesting facts with a higher difficulty level is fine, it is nice to learn a few interesting things (like someone else on another quiz suggested, something about the oldest/biggest tree in the world, forgot the country, pretty sure it was in africa) but very obscure, uninteresting historical facts, that noone is gonna get (or remember afterwards) is not gonna make the quiz better.
It should either be gettable (though that does not mean a 100%, currency is gettable, but I definitely won't get them all), interesting, or a good fact to learn (like the lake chad, embarissingly I had no idea)
I feel that if I did either approach they wouldn't make for very entertaining quizzes, so I try to balance out the difficulties as much as I can. But sometimes, they don't always play out depending on what the quiz topic is. For example, my quiz on James K. Polk ended up with a lot of the answers being states because he spent much of his time in office acquiring more land for the country.