Hang on a second C-H,
The PG (Plastics Gauge) was invented in Italy, so that could explain the wayward sizes.
Another point that Ragnar touches on is this:
  • In some places Electrical conduit gives the inside diameter of the conduit, in other places, it's the outer diameter, it's the same here with water pipe.
  • Plumbers here still use Imperial sizing on pipes, as do Refrigeration Engineers, regardless of wall thickness.

I size the hole-saw to the size of the outer diameter of the thread of the gland.
I've worked on gear to IP68 (Continuously Submurged to 8 metres) and I haven't had a leak yet!. [Linked Image]