Here's some additional information in case anyone is interested.
The 11 ways to get to X are:
0-9 pins on the first throw, followed by a spare, followed by 11 strikes. (10 total possibilities)
11 strikes followed by a gutter ball. (1 possibility)
= 11 ways to get to 290
The 1 way to get to X+1 is:
11 strikes followed by a horrible throw of just 1 pin = 1 way to get to 291
According to this trivia page,
Some people have bowled 291 before, in some cases due to a gross error on the last throw. I'm sure there was laughter involved!
It's much harder to get just 2 pins on a single throw (with 10 pins standing) than to get a strike, so the least likely score in a single game is probably 292. Sometimes 292 is attempted on purpose, basically for bragging rights, since it's so much harder to achieve than a perfect score of 300. (I suppose it's also probably the case that someone attempting to score 292 doesn't succeed and could end up with 291 instead.)
A score of 299.5 has happened before when on the last throw one pin broke in half and part of it remained standing.