You're right, and most programs try to do some sort of gradation instead of a sharp cutoff. Unfortunately, all the programs are different, with cutoffs at diffe

You're right, and most programs try to do some sort of gradation instead of a sharp cutoff. Unfortunately, all the programs are different, with cutoffs at different points, with differing coarseness. A lot of this is related to the problem of making taxes doable by hand, which, several years after the invention of the pocket calculator, could be considered a little outmoded. A lot is also related to the general difficulty of making any changes at all to welfare programs.