Worlds are not fully removed when deleted from within Minecraft

I kept generating lots of new worlds with hopes to get a seed that would show off all the new 1.4.2 stuff easily. After a while it just kept generating worlds that I had already created and then deleted...

For example; On a previous world I made a boat and started going across the ocean to get to a little island that I spotted... about 15 worlds later it generated the same world and spawned me right next to my boat. I don't understand how that is possible if the world was supposed completely gone from my computer.