ELO rating for online games is not calculated in the same way as the official ratings. Here, the system used is called the Glicko system. It considers the rating volatility by calculating a RD parameter (rating deviation or standard deviation).
The bigger the RD parameter is the bigger the number of points won/lost will be. When the players start with a new rating, they are given a rating of 1720 points and a RD of 350.
Their rating will therefore adjust to their actual level due to large variations of points.
After each game, the RD parameter as well as the number of points won/lost decrease.
Just after 5 games, the RD parameter has lost half of its value. Above 100 games, it will vary only a little.
When you do not play for a long time, the RD parameter goes up in order to readjust its rating.
For more information about the calculation: take an aspirin and visit the website of its inventor :)(http://www.glicko.net/glicko/glicko.doc/glicko.html)
