Yeah, most folks don't realize that unless they've been there.
If I get adequate service I double the tax... here tax is 8.25% so doubling = 16.50% and may often round up to keep it even.
Now, if I have a server that goes above adequate then I'll double and then add depending on how awesome there are. IE just a little above and I'll get it up to 20%. REALLY awesome, great attitude, food great (keeping in mind that the kitchen is responsible for the cooking, NOT the servers fault if they botch it when the order was put in correctly), drinks kept up... etc. then 25% isn't unheard of.
Now, I read somewhere that the minimum wage law now had a thing added... that if the servers tips plus wage doesn't equal the regular min. wage that the employer has to make up the difference so that everyone is still getting at least minimum wage on a slow shift... but I could be wrong on that... or maybe it's just Texas... not a server now so I can't say for sure.