To quote SharonfromSXM "People don't want to be charged $2.00 & change at the bar". I have absolutely no problem being charged $2.00 at the bar for a Happy Hour beer and tipping $1.00 to the server it makes me feel good that I just gave a 33% tip. But when I'm charged $2.60 for a Happy Hour beer and in order to reach that 33% by adding .40 cents to the bill it makes me feel cheap.
So if I add additional $1.00 to the $2.60 plus .40 cent bill it makes me feel that I'm paying too much. I just left the bar 15 minutes ago and I haven't met a server that I didn't like.
My math might be wrong but I hope you get my point.