Why mysql not using the index here?
The percentage varies based on a lot of factors; I say "about 20%".

For tiny tables, you would be hard pressed to see any performance difference.

Usually the Optimizer "does the right thing" in deciding between using the index (and bouncing between the index's BTree and the data BTree) versus simply scanning the entire table (and throwing out unwanted rows).

Bottom line:  Don't worry about it.