I think it sounds extremely feminine so I see why people would use it on a girl. It reminds me of names like
Eva (which is pronounced EH-va, like
Evan without the n, in some countries) and also of names like
Eve,
Evie and
Evangeline.
I know two girls named
Evan. I think it fits a girl better, especially with the nn
Evvie or Evvy.
I think the same about
Kyle and
Ryan. They sound very soft.
Still I think they're all fine on boys, too.