Assuming January 2011 as a baseline:
- Jan 2012 had approximately 94% of the site traffic of January 2011
- May 2013 had approximately 210% of the site traffic of January 2011
So the site has more than doubled in size since January 2012, so it seems reasonable that the number of votes has increased as well. Plus I believe most voting happens in the first 48 hours or so, votes for the rest of the week will only trickle in.