I am under the impression that these are basically ways of expressing "Over 9000" in increasingly convoluted ways)
Pretty much.
Numerical values in this game are attached to adjectives. So Athletics 5 is the same as Superb Athletics.
The highest adjective is Legendary, for the number 8. And Legendary is a hard word to top. But sometimes people roll above 8, so...
Anyway, I've seen some post-Legendary adjectives in one of my PbP games.
Arcane uses Awesome for 9, Stupendous for 10, and Superlative for 11.
Hick Jr proposed the following ladder:
+18-Heroic
+17-Godlike
+16-Flawless
+15-Stylish
+14-Awesome
+13-Marvelous
+12-Crazy
+11-Superlative
+10-Ludicrous
+9-Amazing