its the evolution of the hardware/OS as GM went through obd1/1.5/2

the v6 vehicles didn't need as much work to get to pass emissions and didn't get the R&D time that the v8 cars did for mpg/emissions.

the slight change in njector flow rate due to its "perceived" pressure differential (fuel press vs manifold) wasn't as important as them sorting out the 4t65e's drivability

I can only imagine what their breadboards looked like as they went through the major pcm hardware upgrades in just 4 years