Members also need a minimum of 15 posts in order to vote, if I remember correctly.
It's SUPPOSED to work this way but doesn't seem to work in practice for whatever reason.
In the end, yeah we seem to have around the same # of banner votes per month.
On average, the site has an "Active" member base of around 200 people. Meaning the user logs in once every 30 days (minimum). On a daily basis we seem to range from 80-114 (Yesterday was 114). This means up to 114 different member accounts visiting the site in some way yesterday. So if we get 60-80 votes on a contest, that's really only like a third of our monthly visitors or about one days worth.
Obviously a lot of the members just come here to see a custom, or look at the BST or whatever, but it is what it is.