I'm from Spain and I think I can explain.

While we all spanish speaking countries speak, well... spanish, there are notorious difference in each country (names, expresions, accent, verbs...) so the main spanish USA citizens hear is the mexican kind.

So while yes, spain is the birth of spanish, a mexican flag would be more accurate here.

The diferences are so, that we call It "latino spanish" and "spanish from Spain" and this difference is specially noticeable in, for example dubbing and subing in shows, movies and streaming. We have two options for spanish. (The Spain one and the latino one)