In simple terms, files in a directory are not guaranteed to be processed in alphabetic order. Certainly in Windows they are often in the order they were added to a directory. I am not sure about Windows, but certainly in DOS (showing my age here) if you deleted files, then the directory slots could be reused by new files added later.

Anyway, this sort puts the files in filename order - so if the file names start with track numbers they are in track order. Then they are displayed in Unmatched Files in this sequence.