The one thing about Dynamic Queue I'll never understand..

I play solo and pick Fill pretty much all the time, simply for queue times sake. Fill for me is 99% = Support. Now, I don't mind playing that role, but how on earth do I always end up there, while the enemy bot is always premade? Why can't I get a premade bot and Fill lets me get any other role in their team? Can someone explain me, why this algorithm is so messed up? Because I really want to know.