Comment by Lio

Yep, as it is some sizes are easier to work with.

Domestic drill sets don't seem to be designed for tapping holes but if you stick to M3, M6 and M10 the tapping sizes do correspond with the 2.5, 5 and 8.5mm drills[1].

I guess if it was based on a ratio system you would need special tapping drills for all of them.

e.g. M4 needs a special 3.3mm tapping drill already.

1. According to my trusty Zeus tables.