I find it fascinating that it uses a 2-9 scale to grade difficulty.

The rating is described as a rating "1 - 10"

But every trick is actually graded 2 to 9. ( https://libraryofjuggling.com/TricksByDifficulty.html )

Presumably no-one ever wanted to define a grade 1, just in case an easier one was discovered, and similarly for 10.