Deletion of Snapshots by other Snapshots

Whenever I install a snapshot, it's just normal. But when I install another snapshot with the other snapshot in my versions folder, the other snapshot gets deleted after I install the snapshot.

Ex:

I install 14w21b.
I do some stuff in my world.
Then I download a map only for 14w30c.
I install 14w30c.
I play the map normally as I would.
After playing, I go back to 14w21b.
Minecraft Launcher says "Ready to DOWNLOAD & play Minecraft 14w21b.
I check my versions folder, just to see that 14w21b wasn't there anymore.