Type "Instructables Rating System" into the search box. One of the first hits should be Rachel's forum topic with all of the details. You can narrow the search by putting that phrase in quote, and also using "inurl:community" to limit the result to forum topics.

For your specific question, the reported rating is an average between your individual "vote" and the set of all ratings of all kinds on the site. With only a small number of votes, the reported value will be close to the site-wide average (so that a few bad apples can't artificially skew things). As the number of new votes increases, the weight of the site-wide value will decrease and the result will asymptotically approach just the average of the votes.