The theory I've heard is that the more prime a number is, the more random it feels. 13 feels more awkward and weird, and it doesn't come up naturally as often as 2 or 3 do in everyday life. It's rare, so it must be more random! I'll give you the most random number I can think of!

People tend to avoid extremes, too. If you ask for a number between 1 and 10, people tend to pick something in the middle. Somehow, the ordinal values of the range seem less likely.

Additionally, people tend to avoid numbers that are in other ranges. Ask for a number from 1 to 100, and it just feels wrong to pick a number between 1 and 10. They asked for a number between 1 and 100. Not this much smaller range. You don't want to give them a number they can't use. There must be a reason they said 100. I wonder if the human RNG would improve if we started asking for numbers between 21 and 114.