The unicode BOM is code point U+FEFF. The process of encoding it determines the order.

Encoded to UTF-8 it becomes EF BB BF. Encoding to UTF-16 big endian it will become FE FF. Encoding it to UTF-16 little endian it becomes FF FE.

Converting it back from UTF-8 always gives you U+FEFF since UTF-8 doesn't care about endianess. Converting it back from UTF-16 using the correct endianess gives you U+FEFF. Converting it using the wrong endianess gives you U+FFFE which is defined by unicode as a "non character" that should never appear in text.

Makes sense, thanks :)