if it was the wine and wife or grandma, it means the drop was made off camera and in front of everybody. lazy for my tastes. if sansa did it, it means she needed to pick a cup up off the floor in a spot where briefly nobody could see, but if it was the wife or grandma, they never had that opportunity because the only window was very short while they were wheeling out the pie. plus the tyrells have the least to gain, unless there is some new strategy that hasn't been made apparent to the audience yet. the tyrells dont gain much by killing joffrey, but they could gain much by killing both joffrey and tywin if they also had some major secret backers