It is weird, because it works completely differently from how it works on the ground.
It's likely this way so moves like Sindel's air scream or Kitana's air fans work at all.
As a side effect, it also enables you to do predictive anti-air jumps that you cancel into an attack if the opponent didn't jump (Kabal).
And lets you tuck in your toes for someone who otherwise leaves them hanging (Kotal j2 xx air kat).