Maybe I'm an outlier but I like having shirts and pants that fit differently. I think if all the brands fit the same and followed some arbitrary "rule" of how clothes should fit, it'd take the excitement out of trying new clothes. I enjoy that my Flat Head shirts fit very differently than my Iron Heart, Stevenson, Sugar Cane, 3sixteen, etc shirts much like I like having different styles of shirts/jackets/pants.
I disagree that Flat Head's measurements are "wrong" because I've seen plenty of people of all different sizes wear flat head and it looks good. I've talked to many people who prefer the flat head fit versus for instance iron heart. Based on the logic that 'it doesnt fit certain groups of people so it must be wrong" then all brands are all wrong because there is no brand that fits everyone.