You're confusing bolt/nut diameter with thread pitch it sounds like. You can go to the hardware store and buy 18mm bolts with different thread pitch. That's all that is going on. The Japanese manufacturer specified one and the German manufacturer specified another. That is not unique to metric it happens with English/Imperial fasteners too. It's further confused by some systems listing the distance between two threads (e.x. 1.5mm from thread to thread) & the others listing # of threads that fit within a specific measurement (e.x. 16 threads per inch). Campagnolo & Shimano parts are all specified pretty carefully. They're not compatible because they're not designed to be compatible, it has nothing to do with the measurement systems.