I can't tell you why :( It's just a basic Ruby specific feature, not "Obscure" or "Creative" xD !!

This Kata is perfect, but there aren't enough final tests case.

My first code commit was wrong, I think, the test arrays always start with more than 9... so if a code only work with 10 in first index, this code can pass, like mine.
I began to test >= 10 with the first index (in a while), so if array start like that [9,1,10,80], my code returned false but pass in the test cases.