If an awesome article gets 50 "5" votes and then a single "1" vote our thinking is "something's wrong". Not all members play nice so we filter out spurious or malicious votes that are further than a certain deviations from the weighted raw score. The final score displayed is the weighted score using only unfiltered votes.
The actual acceptable deviation from the mean that is used to filter votes is calculated as 1.75 x the standard deviation, and then increased to ensure that voting values on either side of the mean can still be voted.
Filtering only kicks in once there are 10 votes to an article, and due to historical reasons not all ratings can be treated accurately due to our systems only having full rating data since 2003.
Note that filtering of votes only affects the score and popularity. All reputation points awarded (and lost!) through votes still apply, regardless of whether a vote is filtered from the score or not. Also remember that each time you vote the mean and deviation are recalculated and will change. A set of false 1 votes on an article that deserves a 5 will initially have the first few (correcting) 5 votes filtered out, but soon enough when sufficient 5 votes are posted, or members with high enough reputation post a 5 vote, the mean will be corrected and the initial spurious 1 votes will be filtered out in turn.
The calculated mean and accepted deviation are displayed in the rating histogram as μ and σa.