It's simple your crusing along steady speed flat land af 15/1. you come apon a steep hill now the ECM richens the fuel 12 or 13 to 1. It's not leaning out to 12/1 now on the down hill side it will lean out the ratio
Sounds like you may be starting to be receptive to the ECU possibly doing more than just chasing λ = 1 . Great!