A couple things that probably factor in to that number. 1. We've been encroaching on their habitat. 2. They were once very close to extinction, because of hunting from the 1800's to the mid-1900's. Luckily they got on the endangered species list and there has been a great comeback. Not only are they not considered endangered, but their population is extremely healthy now.
The people who raise alligators harvest the eggs and are required to put a percentage of the alligators they raise back in the wild. I saw a video on you tube about this and I believe the alligators that were put back in the wild were released at around four foot long.