Where does 3.15 mm come from?

I needed to drill out a broken-off 2mm dowel pin, and the best tool at hand for spotting and centering the end was a center drill with a 1mm tip, and a 3.15mm shank. I ended up holding the center drill in a 1/8" = 3.175mm collet, which worked fine. But it got me wondering, where does this strange tooling diameter of 3.15mm come from? I have some end-mills that are also 3.15mm. But given that 3mm and 1/8" are so close, why is stuff manufactured to 3.15mm? I have never seen 4.15 or 6.15mm, for example. Can someone please explain?

Cheers, Bruce