It's a hierarchy in that the low end companies they hire many riders with no experience. To get a job at the better firms, they will test your knowledge on routes before you can start and will fire you if you are too slow. These firms are "better" because they hire a smaller amount of riders that are faster at their job so you will earn more than at an "entry level" company because each rider gets more runs and the rate a likley higher as the company charges more for their faster service.

I used to work as a bike messenger in summers at college. Best job I ever had.