It is, Windows NT's adoption of Unicode predates UTF-16 and it's impossible to really make it UTF-16 compliant without breaking obscure corners of compatibility

It is, Windows NT's adoption of Unicode predates UTF-16 and it's impossible to really make it UTF-16 compliant without breaking obscure corners of compatibility. Most programs assume it's all UTF-16, and it happens to work most of the time... at least until you encounter those sequences that are invalid.