There is a whole thread on /. about the pay system failures in US public schools.
As usual with slashdot, you get sometimes a pearl while digging through a multitude of clueless comments. Here is one I found out for you:
Over time you get more and more complex rules for how to calculate pay. The end result is that nobody understands their pay slips anymore and it is nigh impossible to implement and test a system that handles all the exceptional cases.