Because mail fraud cases address a very large spectrum of actual crimes. The typical sentence for a mail fraud case is 21 months; there exists a very large number of aggravating factors that increase mail fraud sentences, including the scale of the operation (if you mass-mail to perpetrate fraud, you'll do more time), the sophistication of the victims (if you try to defraud fixed-income seniors, you'll do more time), sophisticated attempts to conceal your activities to avoid investigators, if you react to investigation by trying to move your whole operation out of US jurisdiction, if you damage the solvency of a bank, things like that.

People fixate on the maximum sentences when they should focus instead on the minimums (here, 10 months). Just by nature of what "mail fraud" investigations represent, you're bound to get wild swings from case to case.