On sizing up in general
I think the reason sizing is so different for people of the same measurements, is because shorts of the same brand, model, and size will differ from pair to pair. I recently stopped in at a shop in the village of Alpe d'Huez, at the start of my descent, to pick up a pair of SaxoBank bibs (89 euros). They were both made by Sportful, in Hungary. I am the same height and weight as a certain Mr. Armstrong. I tried the L's, and they were snug. I went over and picked up the XL's, and believe it or not, they were a tad tighter. I went with the Larges.
My Castelli Velocissimos in the recommended sizing up are Larges, which I replaced with Mediums. So I think the difference is not due to faulty opinion, but experience with two examples of supposedly identical shorts. And that's MY (faulty?) opinion.