What Wolfpup said.
It's hidden, but each map has a different weight that actually changes based on map played history and server time.
For example, it will be easier for badwater to pass than hoodoo if it's 2AM.
The reverse is true as well: in your example, we want some "less-popular/played" maps like frontier and custom ones to win more frequently when it's "prime time" (late afternoon/evening).
During prime time, we'd like to see more map diversity (otherwise our regulars will suicide en masse after playing 6 badwater/goldrush in a row, and we need you guys!) and when it's late at night, we prefer to play more popular maps to make sure we have enough players on the servers (meaning we have to rely on Quickplay and non-regular players outside Americas).
To answer your question, yes, sometimes a map with less votes will win, and even if it doesn't make sense, it's not actually broken. If we followed the vote results all the time, I'm afraid we would be playing the same maps over and over again.