Originally Posted by Wingslime
That's definitely part of it. I would argue that there's also a bit of the fact that if the REST of your team is ahead enough and brick-wally enough that you can survive as a Sivir, Varus, etc, then you were already going to win that game and your choice of carry wasn't really a deciding factor.
Basically it's the same thing as item winrate. You weren't winning because you had X item, you had X item because you were winning.
I think this is little bit silly. Look at the top 5 winrate ADCs.
1. MF - lane bully with an amazing teamfight ult
2. Draven - lane bully that falls off pretty hard if he doesn't snowball.
3. Sivir - Safe pick (shield) with a lot of early long range damage(can be a lane bully) and good teamfight utility.
4. Ezreal - safe pick(extreme range/mobility) that plays both passively and aggressively quite effectively and has good midgame/lategame.
5. Varus - lane bully that has a great teamfight ult(Actually one of the only champs that can outbully a draven early)
All of these champions have a lot of strength potential for early game. You don't see Ashe(teamfights a lot like varus but much weaker lane), Tristana(amazing lategame but all-in early game and no midgame), vayne( amazing lategame but usually weak laner), kog maw (amazing late game but pretty unsafe in and out of lane). The only major early game bully not on there is caitlyn - and she bullies through range and not being a huge damage dealer.