It’s a difference in terms. You could order a chicken sandwich in the US and get something that no European would call a sandwich, but a burger.

In the US, a “burger” typically refers to a ground meat patty (beef, chicken, etc.) served on a bun, while a “sandwich” is more broadly defined as any filling between two slices of bread. In the UK, a burger is generally considered a specific type of sandwich, specifically one with a patty in a bun. While Americans may use the terms somewhat interchangeably, Brits typically distinguish between a burger and a sandwich, with a burger being a subset of the sandwich category