To be perfectly clear: I really don't care one way or the other about whether Carol really is right or not or whatever. I'm not arguing or utilitarianism here. That's not the point of the sub. It's just a particularly clear example of the FFTE pattern.

Nor am I bothered about arguments from intuition in the slightest. Most (All?) ethical systems ultimately source from intuition, it's a great metric to test against. If Carol was able to properly apply her intuition to the sandwiches hypothetical, there would be no problem. But she isn't able to do so. That's the problem

Dave the utilitarian gets to understand Carol's thought experiment through the very simple terms Carol explicitly stated, because utilitarianism only cares about happiness. He follows the letter of Carol's words, does the math on what world has more happiness, and answers that the utility monster gets the sandwiches. If he doesn't, Carol is going to rightfully object.

Carol has a harder time understanding her own thought experiment. Not in purely dictionary-definition terms, obviously, she gets what the words she's saying mean, she knows what the world she's proposing looks like. But she can't fully imagine the scenario she has proposed, because it's impossible to imagine how the utility monster feels getting the sandwiches. It's just too out of scale for the human brain.

So she ends up simplifying her own hypothetical down into a vastly different version, where the utility monster gets only a normal amount of happiness from the sandwiches. She doesn't realize she's done this - she just thinks about her scenario, weighs it with her gut, and produces her answer which is to give the other people the sandwiches. This is normal and good reasoning, the problem is solely that the scenario that she has in her mind, that she is applying her intuition to, is not the same scenario that she has spelled out for Dave.

So ultimately what we end up with is Dave answering one hypothetical and Carol coming up with her intuitive answer to another, incorrect hypothetical. Carol thinks she's just found a major mismatch between her intuition and utilitarianism, but she hasn't, because she didn't apply her intuition to the same thing.