Is it always the adc role that you get a 50% chance.

Every time I get into a rank match I most likely have a 50% chance of getting a bad or good adc on my team. The ones that are know how to farm, when to group up for group fights, and can at least make an impact for the game. I know the other roles you might get a good or bad player, but why is it a 50% chance of getting a good adc? (this applies mostly on vayne, draven, and twitch players)