Nice Japanese

So I'm walking down the hall today, and some guy talkin' on a cell phone says "Hallo" to me. I say "Hi."

Then he says "Nihongo ga jouzu" (<-- You speak excellent Japanese).

HUH!?!

The only take on this I've go is that he thought I said "hai" which is "yes" in Japanese, but which would not be an appropriate answer to "Hallo" and certainly is no indication of one's ability to speak Japanese.