It's your whacky pipe system and you know it
Your daft stuff is in the schedule system and the UK system for plastic pressure pipe is the class system (class C, D, E, T).
Of course the one I was interested in was the magical ID larger than the OD
We used to use the class system, A-F I think it was, you can still find the odd pipe in the class section but we don't use the schedule system either.
The AS/NZS (Aussie/NZ standards) are the PN system, also found in Europe.
So we have a mixture of class, schedule and PN system here, whereas the schedule system is sometimes found on smaller fittings that have been imported from the States, class system from a few years ago, and the now current PN system.
The PN system has very similar dimensions to the schedule system, all your pipe and fittings work with it. Only difference i've noticed is their 1.5" pressure pipe has a small I.D then our 40mm pressure pipe but both have the same O.D.
That's why we have so much golfball barrel pipe, because 40mm is very common, yet in America you need SDR21 pipe.
And 50mm pipe still has a larger I.D(55mm) then it's nominal diameter(50mm).