The site rating actually won't increase as you play more games. You need to win games to increase your rating. If you win a high-rated player your rating increases more than when you win a low rated player. And vice versa, if you loose against a low-rated player your rating drops more than when you loose against a high-rated player.
Ratings are calculated using the Glicko-2 system, designed by Dr. Mark Glickman (Chairman of the USA Ratings comittee). It is actually a very well thought out system and currently the rating system used by the Australian Chess Federation.
What is nice about the Glicko-2 system vs the more common Elo system, is that the Glicko system gives the "range" of your true strength with a 95% confidence. E.g. your strength is between 1150 and 1250, instead of just 1200.
Basically, on a bad day you will "probably" play at a 1150 level, and on a good day at a 1250 level.
In the end ratings is just an estimate of your strength... I've seen a couple of nice upsets on ChessConquest where a low rated player beats a high rated player.