That's fuel-trims causing shift. ECU has pre-programmed maps and target AFR. Using a piggyback to "fool" the ECU by modifying MAP/TPS values tricks it into injecting more or less fuel only temporarily. That's because ECU will analyse O2-sensor feedback to see how mixture is burning. If measured AFR doesn't match what's programmed, ECU will adjust injector duty cycle up or down until it does match.
So it's best to re-program the actual ECU maps themselves.
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