Hi TestingABCD,
If I were a wise man, I would pass on your question and wait for SendofJive to provide a good answer, but I do have to caution you that the lead in sentence to the link you provided states: Note This topic applies to Windows XP, Windows Vista, and Windows 7. and you are running Win 8.1. I do not know if it is different in 8.1 or not, but you need to make sure you are looking at info that pertains to your OS.
I am far from an expert in this UA String area, but I did locate this info:
http://msdn.microsoft.com/en-us/library/ie/hh869301(v=vs.85).aspx
So there may be some differences between Win 8.1 and other OS UA Strings.
As far as your question goes in regards to why the two registry entries are different, my registry shows Current User as IE and shows Local Machine as both IE and Firefox. I am not an expert in the registry area ( I hate to even peek into it), but I can see where there may be a difference if the "current user" has a different default browser.
Hopefully someone much smarter than I can help with an explaination!