• First - Sort by Extension is a mistake?

Question related to mission Sort by Extension

 

sort this list by the file extension, The files with the same extension should be sorted by name. then i think this case is wrong? "sortbyext(['1.cad', '1.bat', '1.aa', '.bat']) == ['.bat', '1.aa', '1.bat', '1.cad']"

why ".bat" is front "1.aa"??