I was going through some old logs and found that my WB reading was spot-on (i.e. same as final fueling base and OL map all the way through to 7K rpm) until a certain day, and after that day it started reading leaner.
I installed an electronic boost controller (HKS EVC) that day.
I'm running only a couple of psi above stock with EVC and I wouldn't have thought that would throw the WB reading off.
Had a look under the hood but I couldn't find any sign of leak after MAF that would make the WB read leaner.
So I don't know. I adjusted the MAF scaling anyway to compensate for it..