GMAIjectors were developed due to two issues.
1) The airflow in the big bore Continental engines are not even. And that due to the pressure pulses, as described, there is fuel injected for one cylinder that gets carried out into the log and into another cylinder. So even with perfect injectors, different cylinders would be at a different fuel-air mixture.
2) At the time, the Continental injectors were NOT consistent in fuel flow. So you could have a great LOP running engine, by chance, and when the injectors were cleaned and put back into different cylinders, it would run horribly. I was around on AVSIG when the first testing and development was occurring the test data on stock injectors was eye opening. So GAMIjectors are also held to very close tolerances and each injector it tested for the actual flow rate. In about 2003, Continental went to a higher tolerance on their fuel injectors and some tuning to even out mixtures. But GAMIjectors are still better.
0.5 gallon per minute difference is not in the injector flow rate, but the fuel flow where each cylinder EGT peaks when leaning. So you start leaning and cylinder A peaks at say 12.0 GPH (total flow for the engine), the other cylinders peak as you lean further, and the last cylinder peaks at 11.5 GPH flow. And it is VERY good for a stock engine. With GAMIjectors and some testing and tuning, you can get the spread to 0.2 or less.
And realize, you cannot just do this to your injectors. The FAA will need to be involved. GAMI has done the work and has the approvals (STC).