bjh -- The St. James was renovated (paint restoration, new carpeting, seating, reconfiguration of restrooms, reconfiguration of lobby, and probably some other stuff I haven't noticed), I believe, following the close of "Swing!" and before "The Producers" came in. That renovation yielded the additional seating, most notably (though not exclusively) via the installation of 15 seats across in each of the center sections instead of the more typical 14.
As an interesting side note, I believe NY State law requires no less than 7 seats to an exit aisle (older theatres with continental seating like the Minskoff are grandfathered in). Somewhere along the line -- I'm guessing when they renovated the Martin Beck in 1996 -- someone at Jujamcyn realized that this law permitted an extra seat in the center section -- i.e. the center seat in a row of 15 seats was still in compliance with the law. Apparently, they decided that the inherent sales difficulties presented by having an odd number of seats in each center row, and the resulting need to use the narrowest possible and least comfortable theatre seats (to fit 15 in the center without losing seats in the side sections as a result) were outweighed by the ability to sell an additional center orchestra, center mezzanine and/or center balcony seat. And thus, the Hirschfeld and the St. James each have the 15 seats across in the center and if they ever re-seat the Kerr, O'Neill, or Wilson (though all have been renovated within the last 15 years so that seems unlikely anytime soon), I would expect those to follow suit.
"No matter how much you want the part, never let 'em see you sweat." -- Old Dry Idea commercial