Maybe they are p2pools?
I saw it, too. GHash and BTC Guild have now together less than 50%...
And solo miners?
I don't know. You have 4300 blocks every month. If you want to hit one block, you need 0.025% of the network hashrate. This is about 3750 Gh/s. 30GH/s cost about 1000$, so you need about 100 000$ of equipment.
-> it's possible, but unlikely. Maybe this are some private pools. 10 people invest each 10 grand and mine together.