the logical explanation: you think you always get AFK's and trolls in your games in your promo's, but you get them in your team aswell during no promos, only you don't make such a big deal of it and forget you had them the next day. During your promos you just notice them because you REALLY want THIS win.
Same with AFKs and trolls in their team: you always think the AFKs are in your team because you notice it, but when the enemy team has a troll or AFK you will think you guys played good and outplayed them, despite the enemy team obviously knowing a teammate is trolling.
You always remember the bad stuff, you never remember the good stuff because they happen more likely.
edit: if you want to check this, take a piece of paper, and everytime somebody flames or trolls you write it down. And classify it as promos/non-promos.