Electronics has a thing for doing pin spacing in decimal fractions of an inch, specified in millimetres. It wasn't so bad when everything was through-hole, because multiples of 0.254mm are messy, but clear. But as surface-mount components got smaller, people started inventing new - increasingly millimetre-based standards. And now it's just confusing. Measure once, check the datasheet three times, send the CAD files off to to the board manufacturer, and discover that *something* doesn't fit in n weeks time because you subconsciously read 'mil' as 'mm'
Enclosures and other aspects of physical design are of course done in metric. Except by USAnians, no doubt.