Let's just put it this way. I put in more effort into my job than most the other people there, but I am not as fast and therefore not as productive money wise. What I do instead is give far higher quality, especially on the cleaning side preventing fines and warnings from the health department. Where they clean a pan in 10 seconds and leave food all over it, I clean it in 20 and leave it spotless. The have 10 more seconds to work on something else, but who is the more productive employee who gets payed more?
Where it gets harder is when you look at sales in general (note: I work as a meat cutter). My fellow workers like to cut steaks and fillets heavy in the hopes of getting more sales by passing it off. Such as cutting a fillet 10oz instead of 8oz and do it in 15 seconds. I take 20 and give them 8 oz when they ask for 8oz. I get more return customers asking for me because I give them what they want, and about 20-30% of the time my co workers have to go back and re-cut, making them take 30 seconds total and losing some of the profit because the 2oz they have to cut off goes to $5/lb burger instead of $28/lb fillet.
All in all, it takes quite a bit of effort on a supervisors part, especially when you consider that we have ~20 people on the meat team and you would have to evaluate each persons performance specifically in depth enough to make that sort of judgement call on effort and productivity=pay. Then how do you determine that sort of thing for supervisors and higher ups?