There are a few different metric standards for "preferred size", ISO, german (DIN), japanese (JIS), and I think there is an American one too.
A German M10 bolt would have 17mm across the flats, but an ISO one would be 16mm. A German flanged M10 would be 15mm - ISO still 16mm.
So an incomplete German wrench set would go 13, 15, 17, 19.
ISO would be 13, 16, 18, 21.
Japanese loves 12 and 14mm, which german cars mostly never use.
My older german cars were 17mm all over the place, but my "new" ones (2000+) use a lot of 16/18, I guess germany has withdrawn the DIN standard on that particular aspect to harmonize with ISO.
So that might explain your "missing" 18mm socket.