Not because of bore diameter. Could it have had original 2 1/2" chambers, later lengthened to 2 3/4" without reproof? 2 1/2" chambers and 1 1/4 oz proof would be very unusual. And the gun was made in 1921, but chamber length wasn't a required proofmark until 1925.
It may have originally been proved at .719 (13/1) which would have gone out of proof at .728. Current quote bores in the article are within correct range for a nominal 12 (but not a nominal 13/1 - That would be my most likely explanation.
The gun is similar in specification to a 'live pigeon' type gun -. and these frequently started life with 2 3/4 chambers, 1 1/4 oz proof.