Well, the difference is the fact that everyone agrees that they're different languages. It's not like comparing English to Scots (is it just a weird dialect of English? Or is it distinct enough to be its own language?), or comparing all the different dialects/languages/[idfk anymore] of Serbo-Croatian to each other, especially if they're all written in the Latin script (in Serbia, you have the option of using the Cyrillic script, but not everyone does it anyway).
