I've noticed this, too. I have a 1984 Trek 610, with double-butted 531 main tubes and unspecified CrMo steel fork blades and stays. It rides stiff and responsive, but with a nice resilience over bumps and light washboard. For a while I had a friend's 1983 Trek 600, same size, 531 double-butted tubes, forks, and stays. The 1983 was a smoother and flexier ride. Bikes had the same geometry.

531 is a little stronger than standard CrMo, perhaps the chainstay and seatstay tubes were a little thinner-walled? That would give more flexibility with equal strength and durability.

As far as "all the other brands produced," I can't say anything. I haven't ridden them all. I have a full 531 db Woodrup, and it is cushy and flexy. Maybe it's just what thin-wall 531 does.