The matchups just seem to go off votes, people upvote champions they think work well in that matchup and downvote ones they think do not. In this case Teemo has 23 thousand upvotes and 11 thousand downvotes while kayle has 7 thousand up and 2 thousand down.
Essentially it's an opinion poll asking thousands of players who has the advantage on the match up. It's very simplistic as it cannot take into account skill levels (for example some champions are very strong at higher ELO but not so much at lower ELO) and may take time to update after nerfs or buffs but it still gives you a rough idea of what people think of the matchup.
In Teemo vs Darius the problem Teemo has is that his auto-attacks are the same range as Darius's Apprehend. That makes it really hard to poke hard enough to take down Darius, especially once Darius gets to the point where a single catch chunks out most of Teemo's health or even kills him completely.
Teemo does have an early harass and poke advantage though, if he can maintain that lead he's fine.