Because the stock injectors don't require modification of the soi offset, therefore the adder still works efficiently as well. Tunes for stock injectors you don't have to change any of the modifiers, adders, or offsets because these are all meant to account for delays or the speed of the injectors. That doesn't change with stock injectors, where as when you install different injectors you have to change the built in delay to account foe the different injection ratio, and speed of the injectors.
Basically think of it like this, if you're dealing with lawn sprinklers, and you have the same heads in the ground, all you are going to change with them is direction, and flow. Now you put different heads in the ground, you're going to have to adjust the spray pattern, but also how long they stay on, and how much water they put out to allow the best coverage and to not over water. Same concept.
Once you move away from stock injectors, all aspects of tuning must be adjusted, even if its just by 5%, to account for the mechanical difference between split shot and single shot. Just the nature of the beast. Where it gets tough is when you can't figure out if its the adder you just changed causing the issue, or that adder interacting with the offset, and then trying to figure out which one to change. That would be my guess as to why many tuners leave the adder stock. Less stuff changed, less things to mess with if the customer is unhappy.
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