They are also the most desirable low-wage jobs as the hours are more consistent. They're challenging to fill because you have to hustle at restaurants, while the average grocery store employee is much less utilized. Bussing tables is a much better gig than folding pants at the Gap, supermarket or whatever -- you easily lose 25-35% of your hours when weather or other variation reduces foot traffic.
example: http://www.aei.org/publication/seattles-new-minimum-wage-law...
That... didn't happen, and it's important to hold predictors accountable if we want to make better predictions in the future.
If you carefully pick the start and end points, you can "prove" a lot of things that just aren't so.