The order of the headers in Message Mode is peculiar!

[rameiko87]

I read on https://www.rfc-editor.org/rfc/rfc5322.html#section-3.6 that 
the order of the headers does not matter when sending emails.
But I can't explain why Message Mode defaults to the following order 
when crafting a new emails:

To:
Subject:
From:

given that the default value of message-header-format-alist is 
different! Is there any reason for this order (and where is such order 
decided?), and do you agree that the default order of the headers 
appearing after C-x m should be the default value of the variable 
message-header-format-alist ?