I think it is for a 73mm wide bottom shell for e-type derailleur for a isis splined MTB-crankset. I f you account for the 2 mm ring of the etype derailleur on the drive side and push in the non drive side the roughly 8mm adjustable (the golden part in your picture) then you have the 10mm difference you measured. (If you don't know what an e type front derailleur is look under google pictures). This is a bottom bracket for a special MTB set up, the chance you will need this is small.