Its an oldie. My Williams wrenches are the same.
1/2" means a 1/2" bolt shaft, but the big square head of said bolt, that the wrench goes on?
Is 15/16".
They used to mark all wrenches this way. But it turned out be easier for mechanics to remember to just mark them the size of the bolt's head.
A 1/2" wrench size now, fits a 5/16 bolt.
I don't really think either are any easier, one system over the other,
but everyone got together and decided with their dollars, buying the wrenches marked for the head size instead of the shaft size.
yours Scott