Well this is not actually a FAQ. It's like a sorta of telling you an answer to one of
the top asked questions.

[ Why are the Singleplayer / Multiplayer files the same ]
When Minecraft was updated to 1.3.2. Mojang decided that Singleplayer worlds
with run on a server that you host LOCALLY [aka on your modem] not public.
Which means it runs with the CLASS files that the servers use. :)