Some facts:

My WB is showing an average of 14.9 (hopefully it's precise). No matter if fuel trims are positive or negative.

When my fuel trims (LTFT + STFT) are positive (average +2%) I do see a lower fuel consumption (5% less). This only on long trips on the highway.
For short trips (like daily home to work during 30 min) the consumption doesn't change.

During decel the WB shows an AFR of 12.5 - 13, LTFT are around +4% (cell # 21)

Your comment?

Can I really chenge the target AFR??
I never touched this value (LS1Edit). I was thinking that was something not to play with...