I have heard that solo mining is useless unless you have a ton of network power. That makes it seem likely the unknown section is not made up of a ton of individuals but is like a private mining pool from just a few locations.
This isn't true, solo mining is just more random. So you will receive big chunks every now and then instead of a steady income.
Solo mining is useless, if you need to sell some of this income to cover electricity costs, because you might not find a block til your next bill.
It's quite possible that there are a lot of people solo mining.