I'm glad to see to see that I'm not alone in experiencing this phenomenon, but sad too, that it is so commonplace.
Since my OP I've realised that this situation mainly arises during playdates with parents from my children's school and thankfully very rarely during get togethers with my usual circle of friends/family. It's probably just a coincidence that it happens with people from the same school but I still have to wonder how people can invite someone over and then not be prepared to offer them a single morsel to eat.
On the flipside, I went to another engagement party on the weekend and the tables were absolutely staggering with the amount of food provided. Despite there being 60+ guests there was still heaps of food left over.
Not sure whether this is a coincidence or whether it was poor planning on the part of the venue/couple, or greed on the part of the guests. A year after attending the engagement party mentioned in the OP, we attended the wedding of the same couple. at a very ritzy and expensive gold club. The food was buffet style and the tables went up in groups of three to be served. We were table 4 and watched as tables 1,2 and 3 went up for the main course and came back with plates piled high. By the time tables 4,5 and 6 got there we were restricted in the amounts we could take as there wasn't much food left. For tables 7, 8 and 9 more food was brought out. At dessert time tables 7, 8, and 9 went first, piled their plates high and, again tables 4, 5 and 6 were restricted. For tables 1,2 and 3 more food was brought out. I was 3 months pregnant and starving, and again, we had to stop at McDonalds on the way home.