Plus, the predators are responsible for notifying law enforcement every time they change their address or employer, etc... Many do not go in and make the changes, so when law enforcement gets around to checking on them, they have to hunt them down. So, oftentimes, the list is not completely accurate.
I used to work in our local Sheriff's office...I worked with the girl who registered sex offenders. It was unbelievable. Of course, you also have those who were maybe 21 and had consensual sex with an 18yo. They will be labeled a sex offender for good (barring unusual circumstances).
What a twisted world sometimes, eh?