Some builders may want a thicker top tube for a touring bike for better load carrying/dent resistance, or may want a heavy tubeset that takes a common 27.2mm seatpost rather than an irritating 27.0mm. Some tubesets also have silly overbuilt fork blades (531DB - not sure if 531C was exactly the same) which would ensure the frame, not the fork, was written off in a front-end collision, and this can be avoided by using someone else's fork blades (Columbus/Tange).
I suspect some builders also worked in total chaos and used whatever was lying around and took their fancy on the day- I've seen photos of Ron Cooper's workshop! The outside diameters of traditional tubes are standardised, so there is no particular reason not to mix and match, since they all fit into the lugs.
The modern stainless steels are very hard and strong. Reports are that they are not fully stainless, if you think you can treat them like titanium and ride an unpainted frame on salted roads. AM-series stainless Moultons can suffer from surface rust, which is probably disappointing if you just dropped £17k on one