Confused about my results.

I've been playing online since mid-July. I've played on nine different sites since then, collecting bonuses. I've played almost exclusively at .25/.50 limit.

At eight of the nine sites I played for only a few weeks--enough time to clear the bonuses and maybe a little more. I stopped playing at each of those sites shortly after clearing the bonuses because I was only a slight winner, and sometimes a slight loser, at each. At one site, the first one I ever signed up at, I've played regularly since mid-July.

My results are confusing. At each of the eight sites that I've played 3-4 weeks at, I am either slightly up or slightly down overall. At the site I've played continuously at, I am a very consistent winner, and am now up over 2000xBB total--about $1,200 total. I have no stats software yet--I've only kept a running total of my results at each site. I've probably played on average about 12-14 hours/week since mid-July.

The site I am a very consistent winner at has a high volume, and in almost all of the online reviews I have seen the competition on the site is rated as tough. Some of the other sites I've played at are rated as much more easy.

So I am basically confused as to how my results can be so dramatically different. There is little to no variation in my play across sites, so I am at least initially inclined to attribute the difference in results to a much-too-small sample from the other eight sites. Yet every time I go back and try one of them for a couple of weeks, I get similar results--tough to win.

So, can someone tell me what is likely to explain this difference in my results?