When a spawner has custom data, (for example, a zombie with gold armor and a sword), the spawner will first spawn a normal zombie, then the next zombie it spawns will have the correct data. Every time you reload a world, the spawner is reset to its original state of first spawning a normal mob, then the correct, edited mob.

Hopefully this video will make the bug more clear:
http://youtu.be/O04rqD2nYTk