Taken directly from the Minecraft wiki:
"When the player dies, they only drop half of their experience as orbs, rounded down to the nearest point. The other half vanishes."
Just died to a swarm of endermen, with 13 levels on me. I respawn at my bed, and run back to the spot. Everything's there.
Except after picking up the XP orbs, guess what? Less than 2/3 of level 1.
I'ma call BS on the wiki.