As a food delivery person, I reported about 5% of my cash tips and about 5% of the other delivery people also reported around the same. However, about 5% of the delivery people I knew reported 100% of their tips and the majority reported all credit card tips plus ~50% of their cash tips. This was in a town where a higher than usual amount of us wanted to cheat the system.

 Now at any big chain, plumbers are going to be forces to report all of the work they do and provide detailed bills (or risk being fired) which leaves not a lot of room for fudging numbers. So self employed plumbers are the only ones that can just not claim cash income like that, which is a steadily declining percentage. Ergo, the numbers are much closer to the provided statistics than you think.