BUT the toal cross-sectional area of the copper is [essentially] the same. For #12 stranded...

Note the sum of the seven strands' cross-sectional areas is closest to the 6530cmil table value. The difference is mostly due to rounding errors, some is due to allowable tolerance. If you back-figure 6530 to a diameter of seven strands, in inches...

SQRT(6530/7) = 30.543mils or 0.030543in

The table value is only 0.030in.

What's to account for??? The overall diameter and circumscribed area is greater compared to that of solid... but it includes "air".

Interesting. My error has been applying Note 5 to "Area" instead of "Overall Area", which is also clearly stated in the note.

By inspection from your diagram, clearly the diameter of a single strand is 1/3 the diameter of the circumscribed circle. If we let A1 be the area of the circumscribed circle and A2 be the combined area of the strands and do the algebra (r=d/2, A=(pi)*r^2), then A1 = (9/7)*A2.

If we look at column 9 on Table 8 (Conductors Overall Area in mm^2) for AWG 8, we see two numbers, one for solid (8.37) and one for stranded (10.76). 10.76 is indeed 9/7 times 8.37. I was under the mistaken impression that the diameter of a solid conductor and that of a stranded conductor of the same AWG were the same (all you electricians stop laughing at the ignorant engineer), but I see now that the overall area of a stranded conductor is 9/7 that of a solid conductor of the same gauge in order to compensate for the gaps.

All of which does not explain the approximately 2% higher DC resistance of stranded conductors over that of solid conductors, but I am not yet ready to concede that it is because of heat transfer because there is no mention of current in the governing parameters of the table and that difference would vary with the current. If the Table stated that the 75 degree conductor temperature was due to the conductor carrying whatever current is necessary to raise it to that temperature over some ambient, so that in a stranded wire the center conductor would have higher resistance due to it being hotter, in turn due to the lower heat transfer happening in the six air gaps around it, then OK, but it doesn't say that. But of course, that may be how the measurements were made...