User blog comment:DocTanner/Why ARAM doesn't feel random/@comment-4652200-20131002141618

There is also another point - queue dodging - if one of the teams get unlucky and get "bad" champions for ARAM or one player gets a champion they are bad with (and nobody wants to switch) - they leave. So that further decreases chances of getting champions bad for ARAM.

Also if you like an unpopular champion like and got a lucky pick there is no guarantee someone won't queue dodge...

BTW very well-written blog with good explanations. :)