I know that the top voter every month gets a free rank. However, if 3 people vote the same amount, who gets chosen?
The top 5 are the people who not only voted the most, but the earliest. So it's not random, more of by the time at which they voted.
The top 5 people who are the quickest to vote on the last day are displayed as the first 5 on the list. So, if you vote the same amount of times as the top 5, but don't vote quick enough to reach such, you won't be getting an upgrade for that month.
Yes as @Prefixed said it's who ever votes the earliest but you still have to vote everyday of the month, I took 2 months to crack when my vote get's reset and for me in the Perth 9capital of Western Australia) timezone it's 8:00.