Whenever an XP Orb is supposed to exist it just doesn't if you're on 1.7, though if you walk by it you do pick it up.

I can confirm the buggy nature of XP orbs on a 1.8.8 server:
Expect behaviour (on 1.8.8) is shown first, then on 1.7.10 in this gif:
https://imgur.com/jTWkUiV

However this seems like an issue with vanilla 1.7.10 Minecraft, as testing in single player also yields the same result.

Yeah this seems to be an issue within the 1.7 client.