A little more love.
This doesn't explain Tony Tough (who happens to be one of my favorite adventure game characters), but for series games like Monkey Island and Simon the Sorcerer, the votes can get distributed amongst the several games.
So, for instance, a person might like all the Monkey Island games, but pick only one because that person doesn't want to use up four slots on their list just for Monkey Island games.
Nancy Drew is another example -- when the final results are posted, you'll often see that (for instance) ten of the Nancy Drew games have 2 or 3 votes each, but that none of the Nancy Drew games will have garnered, say, 20 votes.
I always think that Shadow of Destiny should get more votes, but that's my own "innovative and underappreciated" pick. Its keyboard controls make it less accessible than other games, and you have to play all the endings to really figure out what's going on -- something not every gamer is (understandably) willing to do.