There always seem to be unregistered 4-letter .com domains when I look. When I bought awio.com it was simply unregistered. Taking a sample of 500 names and then saying all of the hundreds of thousands of other names must be registered is just wrong.
I tried it with much larger samples for that character distribution and have yet to find a single unregistered name. You can't say with certainty that it must be that 100% are registered but I think that extrapolating from large random samples is a pretty good way of making inferences.
Faster to just scan the zonefiles. You run into a black swan problem with this technique. However, that said, all of them WERE registered, TWICE throughout history. Once way back and again more recently (someone wrote 2007, sounds about right). People have realized the cost of holding them is higher than the potential sales in many cases, especially at the bottom of the quality barrel. They weren't kept because they sucked but domain investors like all people like patterns and herding and get sucked into poor investments.
