Tricia has it right on. You're handy-dandy IT Person (called formerly as a System Administrator) has rights to just about anything you can think of the network. Your employment information, possibly social security #s, etc. There are many cases of IT people getting irritated at the place they word at then planting software bombs on a server (that basically delete everything on the server) and also identity theft.
In short they're usually underpaid and overworked. Businesses really don't "vest" much into the IT person other than "Make sure it keeps working". About 5 years ago an IT person was RARELY screened.
Now thanks to laws & standards like HIPAA, Sarbanes-Oxley, ISO, etc, that incorporate IT into their audits, by placing a few CEOs into orange jumpsuits really made business open their eyes and realize that IT can be a huge liability (i.e. when I started my last job as a System Admin, I didn't have a background check or anything done on me. The same week I started they gave me the code to the safe were all the rifles and bullets were stored).
Anyhoo, like Tricia said, they're only as good as the polygraph operator. I've only been told of stories from government polygraphs in which the person(s) questioning you will ask you the same question 5 times but in 5 different ways...so it messes with your head