I tell this story to illustrate the maturity level I associate with expecting gifts for extra children.
I used to watch two neighbor boys, 5 and 8, both generally very sweet. However, the younger one was totally convinced that his brother had more birthdays than he did, and he threw a tantrum whenever his brother's birthday was mentioned. I even sat down with the younger one and a calendar to show him the repeating pattern of months and years and where their birthdays were in the year. It didn't matter. He absolutely believed his brother had more birthdays than he did.
His parents rewarded this behavior by always getting him gifts on his brother's birthday. I did not.
They moved away, so I don't know if the kid grew out of it. I hope so. It was barely tolerable behavior for a 5-year-old, not at all acceptable after that.
Possibly someone who he trusted had mentioned that his older brother had had more birthdays than he did (due to him being 3 years older) and he misunderstood that as being per year.
Quite possibly, he is still thinking "nothing existed before I was born," (a common thought in small children), so if his brother has had, say, 6 birthdays when he's only had 3, then his brother *must* have more than one birthday per year, right? It makes good mathematical sense.