Probably special code so that karma you give shows up immediately, that way people don't think it doesn't work. My guess is the delay is shown for everyone else.
This is likely part of a bigger effort to obfuscate karma to avoid spammers exploiting it to appear "liked". Spammer X makes account Y and Z. He makes account Y like Z. On account Y it shows, but not on Z. Spammer then thinks he can't exploit it.
Just a thought.
