Metric bolts come in a graded set of standard sizes based on the diameter of the bolt: 2, 3, 4, 5, 6, 8, 10, 12, 16, 20, 24. They have standardised nut sizes so there are matching gaps in the spanner series. Recently they have changed the size of the 10mm nut from 17mm to 16mm. Most old sets of spanners do not have a 16mm spanner. I am not aware of any other sizes being changed like that but they may have.