The PCM determines how long the fuel injectors will be pulsed on to meet the fuel needs as what the feedback sensors such as the 02 report.

The PCM though needs to command the injectors on at the right time else the fuel could be flowing too early or late to when the sparkplug is being fired on and what position the cylinder cycle is.

The PCM has no way of knowing how slow or fast the internal coil of injector is. Each brand or size of injector's coil reacts differently.

Inside the PCM calibration is a injector offset voltage table related to engine vaccum/MAP and battery voltage.
This is the measured voltage at the PCM and not at the battery since there is some voltage loss.

GM in testing the stock injector knew what the delay time was and added that delay time in mSecs to the table.

Now when the PCM commands an injector on the delay time is added to the pulse width time

The problem is when the voltage changes the PCM has to adjust to assure the proper fuel flow is commanded.

If the car, battery or alt has a problem and the voltage is low, the PCM then has to add more of a delay time to bring the proper fuel volume in control so it is important for you to monitor voltage and know if voltage is lower then of course the PCM is in fact commanding the pulse width on longer which effects fuel trims and performance

Suppose you add a 0.5 millisecond (ms) offset to that table at your normal battery operating voltage.
If the PCM calculates an idle pulsewidth of 2.0 milliseconds before applying the offset, the final pulsewidth after adding the offset will be 2.5 milliseconds.

The absolute change is 0.5 milliseconds, and the relative change caused by the .5 ms offset is 0.5 / 2.0 = 0.25, or 25%. This is a big change!

Now suppose the PCM calculates a WOT pulsewidth of 15.0 milliseconds before applying the same 0.5 ms offset.

After applying the offset the pulsewidth becomes 15.5 milliseconds. In this case, while the absolute change is still 0.5, the relative change caused by the offset is only 0.5/15.0 = 0.03, or 3%. The same offset has almost no affect on large pulsewidths.

Since not all injectors are made equal it is important then when changing injectors is a MUST to correctly change the offset times to assure proper fuel flow and injector being commanded ON when the sparkplug is also fired off.
Demand from the seller they provide the specs for the injector including what the injector offset values are and then have PCM tuned to include the values for the injector offset voltage table

If this is not done the injector can be flowing at an incorrect firing cycle of the cylinder and cause incorrect fuel trim values.

As you notice the chart shows as the voltage is lower the offset delay is larger.