Did you exclude those accounts under control of camillesteemer? I believe he has in excess of 200 accounts in his 15SP down vote bot army. I am unsure if they all down vote everyday, but if they did then the number of:
The average number of unique accounts downvoting in the time frame covered is 290.
Would be a little bit less. One way to eliminate those down votes from the total would be to set the floor for down voters at 20 SP, that would exclude 98% or more of camillesteemer's down vote bot army.
I left everyone in, that way people can do the math for themselves on who they'd like to ignore. That downvote gang being a good one.