Games don't really have a default difficulty when playing on AES- they usually give you a choice of difficulties (although the cursor/selector always starts on "Normal", as far as I'm aware). By "default", I think he means whatever the "MVS" setting in the AES mode defaults/equates to (which I assume is the same as the arcade, with all games).
I don't know that any games have elements that are truly random. Whether computers can create truly random events in software programs, is debatable. There is probably something that triggers a certain sequence of events, in every game that supposedly has random elements (or else it might be one of a number of pre-determined sequences of events, combined with a trigger, or maybe without a trigger). Usually with computers, for something to happen, someone has had to program it first. The people who programmed NAM-1975 might know what triggers the bosses, whether it's a certain number of plays, or something else.
