In my experience, it's also about balance. I could take my old Trek and load the rear up and it would shimmy at higher speed(downhills, etc). I could put on some front panniers and it would have much less shimmy(though it was a flexy 90 520, so still some). Same amount of weight, same placement in the rear, but add front panniers and it rode better and shimmied less. Maybe that has something to do with the pictures you've seen? There's also the factor that seems really strange to me - those bikes may well have shimmy issues, but the person riding them just doesn't seem to care(or possibly even notice???). I actually rode another guy's 520 from the early 90s because we met on tour and were talking bikes. I told him I got rid of my old 520 because it had too much frame flex and I didn't care for the ride. He said his didn't have that problem and asked if I wanted to try it. He toured with more weight than me and his bike was a noodle. Maybe it was the racks too. I don't know. But it was a noodle. I rode it for all of 30 seconds and could tell I would hate touring on it. I thought it felt sloppy and bendy. He said he didn't feel it and was happy with it. Good for him and I'm glad he's happy with his bike. Could be the same for the bikes you see pictures of.