Win at some sites and not other sites

[JPFisher55]

I am generally a winning player (about 2-3BB/100 or 4-6ptBB/100) at $.25/.5 and $.5/1 NLHE. But I find there are some sites that I cannot win at and some that I kill. I cannot win at FT and I played over 30,000 hands there. I cannot win at UB, but I have played maybe 10,000 hands there. But at Vegas Poker 247, Absolute skin, and Wingows, Digital Gaming Network, I win loads of money over alot of hands. At PS and Cake I am about even. I used to love Doyles Room and Pacific Poker. But at Party Poker I was about even.
Granted I tend to leave a poker room if after about 10-15k hands I am not winning; FT was an exception. But still I wonder about the difference because I don't think the play varies that much from site to site. It seems that on the sites that I lose either my hands do not hold up or I am cold decked by KK v. AA or set under set situations. But at the sites that I win money I do not experience such situations nearly as often as the losing sites.
I have no idea why. Does anyone else have similar experiences?

[NU Star]

Your samples are all too small.

Variance.

Also, the play does vary to some extent site to site.