Tipping 180

I had lunch today at PF Changs, a Chinese-esque restaurant the spouse and I are quite fond of. It was early, I needed a quick lunch, and their Lunch Bowl specials were just that. I had the Shrimp with Lobster Sauce and if someone told me I had to have it again for dinner, I'd just smile and hold out my bowl for more. Price for this deliciousness which included soup? $8.50. Add tax and the bill was $9.20.

I looked at it, doubled the tax and rounded up and went to my wallet for $11. I didn't have the correct bills, so I put a twenty in the case and the waiter took it. I figured I'd get back either the exact change, $10.80 in a five, five ones, and coins, or a "round down" to $9, where I get a five and six ones.

That's not what happened. I got back $10, a five and five ones. I counted them three times. What happened to my 80 cents? He just took it as his due? Since when do waiters "round up"?

And why, more importantly, did I feel guilty just slipping a one dollar bill in there? He got the tip I intended, $1.80, but I still felt used.

Has this happened to anyone else?