Spawner's entity display defaults the values of NBT data to basic value, rather than mobs' default.

Here are some examples:

1. Goat in the monster spawner has no horns.
2. Skeletons, strays and wither skeletons in the monster spawner has no weapon.

This is basically the same as spawn mob using "/summon <mob>", and the spawner display using "/summon <mob> <x> <y> <z> {}"; this is probably because the spawn data is required to have the id set.