why so incredibly rare to get teemo in aram

I don't understand why I play hundreds of games and get every character so often but for some reason teemo and a few other characters rarely ever become available. Teemo is a really popular character so I have to imagine that most people have him, yet he is really rare to get in aram. wtf.. it realy seems like aram is not random at all. 90% of the time you get the public characters and then the rest of the time you get every character but a few.