I don't really have any idea, but assuming the length and splines are correct, I suspect that they are simply from different production runs, possibly in different countries. After all if you include S3 swb, interchangeable axles were used from 1958 to around 1980, with production in several countries, even without considering third party replacements.
Since these axles almost invariably fail at or adjacent to the splines, the strength is almost entirely dependent on the metallurgy (composition and heat treatment) and finish of the machining, and any increased diameter irrelevant.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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