pretty much since someone bought up M(x) as a head size...
M(x) is a bolt shank size (aka a touch larger than the major diameter of the thread) and in the metric (and the UNC/UNF) world of bolts is absolutely nothing to do with head size.
Now if were were talking BSF, BSC or BSW that'd be different, in those thread sizes the AF size of the head is directly calculated from the thread size.

