I would say 18% is the standard tip as that is what most restaurants (in GA at least) will charge as "automatic gratuity" for parties of 6 or more. The math on 20% is easier so a lot of people default to 20%
I always thought that 18% is standard for 6+ parties because 6+ parties are much harder to deal with and so require a higher tip. It seems a little silly that they would require a "normal" tip
