I'm planning a new build and for the first time, I'm working with a frame that requires an Italian bottom bracket. My question isn't specific to the build, but a general question about chainline.
Given the same crankset, say Shimano FC-6400, the corresponding English BB would have a 113 mm symmetric spindle (i.e. 3H or D-3H), while the corresponding Italian BB would have a 115 symmetric spindle (i.e. 5H or D-5H).
Why would the spindle lengths be different at all? Why wouldn't the Italian BB spindle have the bearing shoulders further apart by 2 mm while keeping the 113 mm length? Would the chainline not increase by 1 mm in this case? If so, how should this be accounted for?