I have a 2x9 on my Sirrus. Crankset is FSA Vero. It was getting noisy so thought about replacing BB with UN55. From my research, the current BB was RPM, 110.5mm. I also read that 110 was common for 2 ring, and 113 common for 3 ring. That is why I bought the 110mm version of the UB55. When I removed the BB it was marked 113. Measurements indicated it was in fact 113mm.

Why is the UN55 measuring 112mm? What would the 113 mm version UN55 measure? Are these numbers more arbitrary than we make them to be?