stackexchange.com/sites was just updated today to use a new formula for traffic. Area 51 is still using the old formula, but will be updated soon to use the new formula.
New Formula: Average # of Visits each day for the past 14 days as recorded by Google Analytics
Old Formula: Total # Question Views / Total # of Days as recorded in the API
The old formula wasn't very good at all because (1) it only counts question views and (2) it's averaged for all time, so it doesn't reflect where the site is now. So sites that have grown were showing lower numbers than they should, and sites that have shrunk were showing higher numbers than they actually have now.
Update: Area 51 is now using the new formula