The traditional tipping rules dictated that the tip was on the meal and not the tax or the alcohol. The sommilier got a separate tip after the wine was accepted by the dining party. Under those older rules a 20-25% tip may have made sense.

In states with high sales tax (WA is one an our rate is about 9%) the bill can get distorted to some degree. A $100 dinner tab and 20% tip is alot different that a $100 dinner, $30 in drinks and 9% sales tax. The $20/20% meal tip is now 14% of the total bill.