Rules of Proof show 13 as .710, so quite correct.
" " " " 12 " .729.. so presumably a 13 goes "out of proof" when/if it measures at the next gauge upwards.
Also not unknown for different gunsmiths to get a variation of .002 +/_ when measuring the same gun! (Been my experience anyway). I don't know which way this variation of measurement works - as the vendor may say "just in" and the buyer's adviser may say "just out" of proof !