A plain text file does not in any way save what encoding it's in anywhere. Any program that purportedly tells you what encoding a file is in is by definition only giving you its best guess based on the content of the file. Now, since a file which contains only characters which are present in ASCII and is saved as UTF-8 is indistinguishable from a pure ASCII file, either answer is valid. Even Latin-1 and a large number of other answers would be valid.
So the answer why that program randomly outputs one or the other is because its detection algorithm triggers one or the other based on some characteristics of the file content. Only the program author can tell you exactly why. The file is encoded as UTF-8 without BOM. Whatever any application tells you it thinks it is is entirely up to that application.