In many cases like that, where an otherwise metric apparatus has an inexplicable SAE component, it's because that component became a commodity part in Britain or America before the rest of the world industrialized.

Give an example made in Austria: Rotax 9-series aircraft engines are metric, they're held together with metric fasteners, cylinder bores are given in millimeters, etc. It was designed in metric. The prop flange is designed to take three different bolt patterns, 75mm, 80mm, and 4 inch. Because a lot of Rotax engines were going to be sold in America, the land of McCauley, Sensenich and Hartzel. We've been tooled up for 4 inch prop flanges for a century now.