When you try to spawn a skull of a player that has only digits in their name it will not give the player's skull, but a normal one instead.

For example: /give (player) minecraft:skull 1 3

This will spawn in Dinnerbone's head, but when you try /give (player) minecraft:skull 1 3

it will give you not my head, but a regular one instead. This appears to happen to every player that only has digits in their name.