It all depends on where you're standing
Amateur websites - like KatVixen.com - somehow became Solo Girl websites, meaning the site is based on one female. Personally (and this is all speculation) I think this was because the think is that the female was not truly an amateur if she's not running her own site (or having her husband/boyfriend run it) So when Lightspeed & Tawnee Stone came along, she was not an amateur & a new "niche" was created: The Solo Girl
Now, solo can also mean nude and/or alone - that's where the toy & nude categories come into play.
Then there's the oddest niche I've every come across: Solo Gay Men. I think it's odd because you've never seen a Solo Lesbian Women category...or site! There's a tangent on this subject about (stereotype) flaming gay sites VS (stereotype) dyke mullet lesbian sites, but that's a discussion for another time.