Thank you, Robert, for this explanation.

It seems like the opposite of a low PW adder. I'm only guessing but it hints that Multec injectors may to take longer to stop spraying fuel when fuel pressure is low. In order to tighten emissions and reduce the need for INT / BLM to try and correct for this issue the engineers built a function that tried to account for the error and close the injector early. If true then the low MAP qualification is used in place of actual fuel pressure measurements so increasing / decreasing pressure via AFPR would alter the needed corrections.

Curiously when early TPI V8 engines were switched to Multec injectors by a tech following a GM bulletin there was no new calibration supplied and no correction was included. It would be interesting to have early Bosh and Multec equipped engines side by side to compare data.