ECMs contain a base Air/Fuel & timing table stored in the ECMs ROM. It will be there even if power is removed. It doesn't matter if this is the OEM map or one from Hypertech.
There is also some AFR customization that occurs depending on data the ECM received from sensors (MAF, O2, TP sensor etc.). This is what gets reset after disconnecting the battery. This is needed to compensate for dynamic variables because every engine is different and experiences numerous changes throughout it's lifespan.
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