The movement measurement was necessarily approximate and to some degree subjective, but with my backside on the seat and a five to six cm gap between my left and right sides, to the arm rest, that gap was being closed by the oscillation on each side!
I, too, wondered about the "they all do that" comment; interestingly enough I had my airmedical (for my PPL) today and my very experienced man who looks after countless commercials etc and flies as a passenger long-haul often (including 777s) says he's experienced it and "it's usual".
There was, incidentally, no roll apparent when this was happening, but then it was happening for most of the flight!