I dont particularly like Stephanie in this - or more specifically the way she communicates with people - BUT it seems like the 6 in each team had to compete in one of the tasks (we were not told this rule, but you could tell from how people were talking - otherwise they would have sidelined the perceived weakest players from all the rounds). It is possible that she chose not to play Fatima earlier because she knew Willy could throw a dart and thought Fatima's all-round skills (including strenght, determination, competitiveness) would be useful later. She might have tried to pair people who she felt might be strong and weak in each task. for instance, if she picked fatima for the memory task, it would have been lorraine and sinitta in the last task - entertaining perhaps, but unlikely to have done as well as they did.
Just a theory - do you think it's possible?