Explain (in simple terms ) Computer Has to Re-learn after the 12volt battery is disconnected.

On some days I get this in a Gen 2 with a Toyota battery with an engine that's wore slap out with 300,000 mi so I don't know exactly what all has to relearn per se it depends I think also how long the 12 volt is disconnected that may be another thing I would think just plugging in a nexcel battery and starting to drive. I was in 25 35 mi everything should be starting to square up I disconnect I used to disconnect my 12 volt all the time and it didn't seem much was being relearned The minute I hooked the 12 volt back up I would beat my mileage wouldn't even change on my MFD it's always 47.8 I get that with the car running poorly. When the battery was bad and I was running on the ice full time I was getting 39 37 to 39 consistently running the hell out of the car