Just as in real life, voting may not be representative of the whole story. Selection bias can occur where people who care very much about a topic would vote for it while most casual users won't even care enough to vote. The polls I've seen on Reddit are a vast minority of overall votes compared to the subreddit size in subscribers, keeping in mind that many lurkers also don't even directly subscribe to a sub but may simply follow them on /r/all, like /r/pics or /r/politics for example. I know I don't follow them but I see them on /r/all and will sometimes click into the comments.