What is ironic in the line, 'I never eat anything for luncheon' in W. Somerset Maugham's "The Luncheon"?

This is dramatic irony. It is based on the strong contrast between what the woman keeps saying and what she actually does. The narrator is fooled into taking the woman to the expensive Foyot's restaurant because he thinks she is telling the truth when she says, "I never eat anything for luncheon." What is she thinking when she says this? She must mean that she is trying to lose weight and has been sincerely trying "not to eat anything for luncheon." She may be telling the truth, but, like many people who have a weight problem, when she actually does start eating she finds it difficult to stop. Foyot's was undoubtedly a tempting establishment full of the odors of rich food. Once the woman saw all the important people eating delicious and expensive delicacies, she could have easily have gotten carried away. Although she keeps repeating, "I never eat anything," she keeps ordering more and more. And because she really believes she is a light eater, she orders everything a la carte, which makes everything more expensive. The other part of the dramatic irony is that the young man keeps smiling as she continues to order more delicacies, when the reader knows how much he must be suffering as the bill mounts higher and higher.

What is the difference between "lunch" and "luncheon." "Luncheon" sounds light, whereas "lunch" might be a full-course meal. Somerset Maugham may have meant the title of his story to be ironic when he called it "The Luncheon," and it turned out to be something of an eating binge.