The differece is aftermarket vs. OEM. Aftermarket will send out a lower signal meant to correspond with the injector size difference from stock to bigger. OEM takes care of the calculations in the PCM software. Makes sense, so you can use one part number over a variety of applications interchangably. The fewer individual part numbers you need to manufacture, the cheaper it will be. Flashing the PCM software is a much easier way of controlling that variable, instead of putting it in hardware. I know for sure that Ford doesn't manufacture anywhere near the number of MAF P/N's required to have a different setup for each engine/injector configuration.
I'm not sure if this is totally correct, but it is my understanding of the situation. If any Ford types (like Rara or someone else in the know from an OEM standpoint) wants to tell me I'm off base, then I'll believe them no questions.
