Thank you to both Weero and Oliver.

Oliver has answered the question for me. I have witnessed NW, US, and in some cases AA op-upping their elites if there is space available in a premium cabin on domestic (and some international) routes. The difference I am referring to is these airlines do one of two things. They either A) Fully cater the premium cabins, or B) Under-cater and pray for no-shows. If they fully cater the flight, they want those seats filled so the meals do not go to waste. However, if they under-cater, they run the risk of a certain choice being out of stock by the time they reach row 5. We find it efficient not to fully cater unless it is called for (by bookings); rather we cater to how full we are. This is why although we may have open seats in Business, we will not do op-ups due to lack of meals.

Also, Weero, my confusion was to how DL and AF had been cited as more domestic than LH. No harm, no foul