Whats the story with sizes on plug sockets?

[Barreti]

Something I've noticed a few times but has finally got on my nerves which I'm hoping the collective can explain for me.

I have 2 dedicated plug sockets in my old Draper socket set
They have embossed on them:

SKT FOR SPK PLUG 10mm
SKT FOR SPK PLUG 14mm

Only they aren't 10 and 14mm if measured against other sockets.

They are actually
SKT FOR SPK PLUG 10mm = 16mm 'normal' socket size
SKT FOR SPK PLUG 14mm = 21mm 'normal' socket size

So why are they labelled as they are?

ta.
IanB

[Barreti]

Thanks for your help guys. Another of lifes irritations fixed

If only everything was so simple eh