vocabulary - What is the reason that American English and British English use "Post" and "Mail" with different frequencies?

I feel the consensus opinion is that

(1) it's possibly/probably true that "mail" is used more - in general - in the USA than in Britain. I really feel that's about all you can say about usage in bre/ame in this case.

(2) the specific, clean 'reversal' you point out (mail/post on one side, post/mail on the other) is probably spurious; it does not exist.

(3) it's very unlikely there would be a clean explanation of any difference in usage of the words; fwiw nobody at hand has one or could find one :O

I fear that's it!