Why is it the fiddle that it is? I guess the physics is that if the length of the string from the nut to the 12th fret is the same as the length from the 12th fret to the bridge saddle then the intonation should be spot on. However the reality is that simply doesn't appear to be true. I have an old Strat that if you set up, say, the A string that way it is noticeably sharp at the 12th fret and even worse at the higher frets. The saddle actually needs quite a bit of adjustment to get the intonation correct. And yes I did measure with the string held down at the 12th. I'm guessing the width of the fret wire and the radius of the saddle all contribute?
I play guitar because I enjoy it rather than because I’m any good at it