4 min vs. 4 min and 30 secs, to me, hardly constitutes an appreciable difference. I also said, 'ave for an obvious difference [...] of size or thickness' - but this still doesn't mean much re 'every piece of meat is different'. It's pretty obvious one makes adjustments based on a criterion (or on criteria) such as size and/or thickness - your college courses were no doubt accurate.
All that notwithstanding, this usually is not what is meant when one sees the 'every piece is different' phrase bandied about on Q boards (though perhaps it was all you were referring to). One often sees terms like 'bad' and 'stubborn' used to describe a piece of meat that didn't finish like the cook thought it would.
Uh-uh. Meat production - again, especially pork and poultry production - has been so standardized in the past few decades that appreciable differences in muscle structure, fat content, and marbling/connective tissue are virtually unheard of. Though I avoid buying industrial pork and chicken, I have purchased either on hundred of occasions, from wholesalers and retail stores all over the country (I have been cooking nearly 40 years, most of them professionally, as Exec Chef or Chef de Cuisine on both coasts; the past 20 years I've cooked privately all over the country). Having cooked several to upwards of a couple dozen of the same cuts in standard commercial appliances (large conventional or convection ovens, cook-and-hold ovens, pizza ovens), on numerous occasions, one would think that if there were such a thing as 'bad' or 'stubborn' I would have run across it - at least once. Never have.
But this is easily explained because a) the cuts are virtually the same, save for any minor differences in size that are easily compensated for, b) the equipment used creates a fairly static cooking environment, and c) one makes the environment more static - or more even over the course of the cook - by, say, rotating the roasting pans, or starting the smaller cuts later.
This last point is important because it is the thing most Q cooks don't do, won't do, or can't do - the first, because they might not know how; the second, because they're under the impression that they should never 'mess with the meat'; the latter, because with many cookers - or many cooking circumstances (pits in the ground are an excellent example) - it is simply not easy or downright impossible.
I've done 200 pounds of butts in commercial kitchens and, with compensating adjustments, had them all hit done at virtually the same time. I've done 200 pounds in the ground, like you, and had a similar experience to yours (I learned to make one of the butts a larger 'test butt'; when it's tender/pullable, all others are much more likely to be as well). It's not that the cuts were all that different in the pit cook. It's that the cooking dynamics were different - different within the cooking environment, across the pit and top to bottom - and thus the varying finish results.
Whether in ground or above, a WSM or a stick burner, cooking outside allows for a less static environment, and for greater (or at least more varied) impacts on the laws of heat transfer, et al., that you note.
