I don't doubt that the better waitstaff average at least 20%.
However, that leaves half-or-more of the waitstaff who don't.
Yes, but what is your point?
Some people make more money than others in all occupations.
“....that leaves half-or-more of the waitstaff who don’t.”
Waiting is also something that one climbs a ladder for, as in s/he starts at a lower-end chain, moves to a more upscale chain, to a ‘better’ restaurant, white table cloth, etc. Along the line they’ve picked up a professionalism. If they’re still at the lower end chain after several years, they are probably not getting the 20%, OR they aren’t looking at waiting as a professional, potentially high-paying job.
By the time a server reach that professional level, his patrons are paying a large dinner/drink tab. And, presuming they’re well-heeled enough to afford that luxury dinner, they’re savvy enough to know how to tip well for good service. High-end restaurant don’t offer anything but excellent service. Or the server is gone.