Yeah, I knew that the number on the vehicle isn't the vehicle weight but the bridge load capacity, so it does make sense for softer terrain to be classed as a higher loading.
Or should it be the other way round..?
Ooh, Warpaint Vol.4 by Dick Taylor has a whole section on bridge classification 'The simplicity of the concept guaranteed its longevity; exactly the same system is in use with the British - and NATO - forces to this day.'
Ooh,
'Because the lowest military Bridge Class was 5 tons, some vehicles lighter than this either seem to have used the 5 marking irrespective of their true class, or did not mark the disc at all.'
Weight classes -
4 Daimler Dingo, Humber Scout Car, Universal Carriers
5 Most 15cwt trucks, Daimler Dingo (later mks maybe?), White Scout, Austin K2 (there she is!), Universal Carriers
So , crossover between 4 an 5 tons. And , apparently also, some vehicles wer uprated to compensate for all the extra gear being carried
Now it makes sense. Thanks Jakko. Knew I had it in a book somewhere!