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So...in this case it was originally under-bored (13) and as they drill the barrels and develop it into a 12 gauge some of the steel is removed?

The bore size was normally stamped by the proof house. They neiter add nor remove metal in the bore, but simply stamp what it IS. Realize that at the time these gauge sizes were marked checking was done with a plug gage & the bbl was marked per the largest size which would enter the bore to the specified distance (Usually about 9"). Nothing more was done to the bore except perhaps some final polishing. "IF" the bore which the proof house marked as a 13 had been enlarged to 12 it would have been Out of Proof when sold new, so very doubtful this would have been done. The bore size does not depict the actual shell the gun is chambered for. I have one old English gun with the bbls marked 14. It is old enough according to the proof marks to predate either the chamber marking or fractional sizes,as 13/1 for instance. It is quite pitted but the bores currently measure just under a 13 size & may be how it was made to begin with.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra