What must really sting is the realization that Elly rarely gets her facts straight. I know that I like to talk about the "I QUIT MOTHERHOOD!!" series a lot but I think that it's germane to the discussion. This is because Elly only thought that April had flipped her off because she wanted to see it. It's like how she used to insist that Mike's simply wanting to know what Mommy was saying meant that he was trying to be a defiant little jerk pushing her buttons; she wanted to see that so that's what she saw. Simply put, Elly wants to see signs that Ted is a jerk because she's a belligerent yahoo who likes to be angry. She could no more give Ted the benefit of the doubt than she could publicly admit that she goes into a blind rage for the sake of going into a blind rage. When you combine this with her child-like inability to understand adult relationships and social norms that get in the way of her winning and her need to nail herself to her cross, what generally happens is that John has to figure out what she means when she spouts her belligerent hokum.
This is why marrying a dullard who doesn't like to think comes in handy. Anyone who knows John can tell you that the person who confirms his biases for him stands the best chance of being believed. Since he thinks that Ted can't be taken seriously because he didn't do what THEY said and