I don't know about "high class" as applied to indies, but I'm signed up with two London agencies which are obviously sooooo high class you can barely look at them without hurting your neck.
Or so the websites say! As far as agencies go, I don't think I've come across many that DON'T call themselves Really High Class with the Most Beguiling Ladies Ever - so I'm guessing it's just a silly marketing term and means nothing much! Apart from the price, which the others who posted above obviously mentioned. But agency owners just want to make as much cash as possible, so they obviously aren't going to prioritize being honest about things, unlike indies who don't really need to boast.
I think it's a bit of a shame, because if you go about high-classing yourself to bits then yeah, I reckon you do mostly attract the jolly wealthy clients - who, in my experience, are more boring and useless in bed and self-obsessed/insecure than your lovely average man! Also, you just don't get to be yourself as much, or do things totally your way.
I've turned this into another of my rants about my bloody agencies so I'll stop now!