I can only imagine someone trying to explain the logic on this one in some ad copy somewhere. "TSO" is the kind of name that requires its own FAQ entry just to explain, and even then it doesn't make sense.
i actually liked "MEGA". the acronym was meh, but "Microsoft Mega" has a nice ring to it. probably would have gotten some flack at release (sega mega drive, etc) but overall i think the best out of those listed.