When a desired task is reasonably easy to do, so that several different users can all do it, the person who can do it in the least time gets the contract. That person bids the correct time it'd take, rounded up to the next multiple of 10 seconds, and nobody can under-bid that person (except under penalty of 10% charge for failure to complete task within the allowed time). Anyone who bids higher, in order to get an unfairly high payment, will be underbid by a more honest user, thus not get the contract. Hence all such contracts are executed for a "fair" payment, effectivly everyone working at the legal minimum wage, by whoever is most efficient at the easy task.
But when a task requires a high skill level, so that only one person (during the time span allowed for bids) is able to bid at all (except for frivolous 10% penalty), that one person can bid a longer time than the task will actually take, providing that the bid is no larger than the maximum bid allowed (as specified in the RFB). With no bidding competition, that user can thus have the lowest (only) bid at such an inflated time, thus can in effect get paid at minimum wage for more time than the task will actually require, thus in effect can get paid for the exact time at a higher rate than minimum wage.
Note that it's impossible to get paid for less time than a task actually took, because the computer keeps track of elapsed time from when the user finalizes acceptance of the contract until the completed work is submitted back to the server. If the user requires more time than the contract bid, the completed work is rejected by the server, and the user is not paid at all.
Thus depending on supply-and-demand, the effective pay rate varies upward from minimum wage, but never drops below the minimum wage. TOP