6pm is early?

When I was leaving office this evening, my boss came out from the pantry and saw my back and he called out to me aloud.

"Uh no...something about work that he wanted to talk about? I don't want to be late for my appointment..."

I turned back and walk towards him.

Boss: "Wah, you're leaving early ah..."

I check my watch. It's 6pm! Didn't know what to say, I simply say whatever that come to the lips (not going through the brain)

Me: "No lar...where got early. Already OT lor"

Having nothing else to say, I turned and walk my way out to the stairs.

For your information, my working hours is from 8am to 5.15pm.

Mr. Boss, should I stay in office and play Wii in the clubhouse until 7pm then only go home? Just to show face that I'm staying late in the office?
Should I purposely come into the office at 9am so that when I go home at 7pm you won't say that I go home early even though my duration in the office is still the same?
Why is it that when I come into the office at 7am, you never ask me to go back at 4.15pm?

Wa bosong liao...