I said a hamburger is sometimes a sandwich, and what I meant is the following: it is empirically a sandwich, but linguistically is not. That is, almost any definition of sandwich is going to include hamburgers, but I still wouldn't refer to a hamburger as a sandwich. It's like, I don't know - maybe that any scientific definition of "animal" includes humans, but that in conversation we actually use "animal" for everything in that category that isn't human. Not a prefect analogy, but something like that.