My guess is that they don't want to look bad in front of you. I bet they are feeling guilty for eating less healthy foods. I always feel like a pig if I am eating pizza and the person next to me just gets salad!
I went to visit a friend of mine a week or so ago, and when we went out to dinner I felt better about myself when I ate less than her. That's not to say that I didn't eat enough to fill me up, but I have always felt huge around this friend because most of our lives I have been eating twice as much as her at every meal we had together. I guess it was something of a portion control victory for me to eat less than her!
Anyway, I bet they feel like THEY are on display, because now they have to compare their diets to your healthy eating!