Maybe the Queen of Thorns was trying to poison someone else (so the obvious hints at her being Joffrey's poisoner are just misdirection but not completely imaginary stuff), but someone other than her managed to poison Joffrey first and whoever QoT was trying to poison was saved in the aftermath of Joffrey's poisoning (at that point she'd probably be wise enough to delay her poisoning, as two possibly important characters dying at the same time would make the Tyrells too suspicious). Or both were trying to kill Joffrey and maybe the possible mystery poisoner just made it first before QoT, so while she might've had the intent of killing Joffrey, she didn't actually do it and someone else did. Those are the only ways it would make sense for Queen of Thorns to not be the actual poisoner of Joffrey.
Or it could be that there are simply some surprising factors behind the poisoning and not just the Tyrells wanting to save Margaery from this monster of a human being, like say Tywin being a part of it all after noticing Joffrey being a horrible person & king. or something
Actually... Maerg was drinking from the same cup, as Mace kept pointing out afterwards. It would have been incredibly risky for Ollena to put the poison in the cup without at least telling her granddaughter "You really don't want to drink out of that from now on". There's a much simpler solution, she gives Maergery the poison. She has much better access to the cup anyway, she'd have way more chances to drop it in.
That scene where Sansa goes to lunch with the two of them makes it clear they're working together on the whole 'should she marry Joff' thing. Maybe this was their conclusion.
The show seems to contradict this with that scene with the two of them last episode but I could see it having happened in the books.