It's not really a theory. Just 30 years of experience. The job has to get done regardless of sickness, vacations, appointments, etc, etc. So a business that needs 200 man hours per week is likely to hire 220 man hours. Maybe less since the economic downturn since times were lean but I've never seen a business that was understaffed with competent workers. By and large, American workers are incompetent and lazy. They want to get paid a lot for a little effort.
Are you incompetent, lazy or both?