Could be because you're using Optifine...
Optifine 'gives' you a cloak if you donate... not sure if you have donated to Optifine or not but I assume that you have
If you look at the optifine post you'll see they mention this
Donate to OptiFine[/b] and receive the OptiFine cape[/b] as a sign of your awesomeness.
The cape is visible to everyone using OptiFine. Thank you for being awsome.
I'm not sure why you don't have the standard 'OF' cape