There is a certain amount of social pressure to give equivalent gifts, and possibly a sense of embarrassment if the gift you give doesn't measure up. My mom used to get pretty angry because our aunt would buy my brother and I almost the exact same thing my mom was buying our same-age cousins - but our aunt would buy a version that was just a bit better. Our cousins had birthdays just barely before our own, so she was pretty sure our aunt was going out and buying the things after seeing what Mom got our cousins. Mom's example was "I bought your Boy Cousin two pieces of a set. She bought your brother three." She thought Aunt was trying to one-up her. In retrospect, I think Aunt was trying to be generous, not trying to one-up my mother.
I imagine you are right, but even so this illustrates that people are sensitive about gifts -- even if they don't feel one-upped or ascribe motives to the giver, they worry that they were being cheap. Or if they are the ones who give a bigger gift, they might worry that they make the other person feel one-upped or cheap.
The point being that there are understandable reasons for wanting to know what others in your circle of family or friends gave your child beyond snooping or gossip, and this is one of them. People want information so that when it is their turn to give, they can feel confident that they are in a range that is comfortable for all concerned.