There is no reason to choose hero first unless they use that data to find a matchup.
Think about it. what other versus game do you choose hero/character first... ie online fighting games .. nope choose after being paired. dota is multiplayer but still you choose after matched up.
Ask those blizz devs on twitter about it. tell them to make choosing hero AFTER matchup before mulligan. easy fix they will never do because then they cant rig the ladder.
everyone gets 30 sec to choose their Hero AFTER finding a matchup. Then you would see much different results.