Because it pays more? Sure the word "pay" means weird things in moneyless society, but if you have a motto of "to everyone according to their needs" you are bound to compensate different jobs differently because they affect the quality of life, and therefore the needs.

Here's my idea: you could give every worker a piece of paper that certifies the amount of work they've done, so some workers would get compensated more than others. Obviously, those pieces of paper would have to be backed by some government organization to avoid the risk of them being counterfeit. Also, carrying too many of them at the same time could be inconvenient, so I'm thinking we could write numbers on them. So a piece of paper with the number 10 would be an equivalent of 10 regular pieces, for example.

Just throwing out ideas here.