I agree the numbers don't add up. AFAIK
they refer to the cells as 2.2V but the battery as 12.6. SO digging out the old GE Wet Lead handbook...you are right, they are nominally rated at 2.1 VDC each cell, with a potential of 2.05 to 2.40 volts while charging, and 2.3 to 2.8V while overcharging and gassing at the terminals. But that's for "pure" Pb or PbO2 versus H2SO4 chemistry, and since most batteries add something else into the lead for a variety of reasons, that's also "just" a textbook battery, probably at a textbook 68F/20C also.
GE used to give the book out for free, their part # BBD-OEM-237, to help people spec installations of GE lead batteries.