I’m not an aerospace mechanic, but I do have some insight.

The formula in the image is incorrect. It depicts 7/16" - 10 cents = 10 mm, not plus. Notice that 7/16" indicates the gap in the wrench, and the dime makes that gap smaller.

Now that that is out of the way, it seems that a dime is 1.35 mm (I love that American currency is specified in metric). So, 7/16" - 10 cents = 9.7625 mm. So, pretty damn close to 10 mm.