the MAF sensor determines how much air is entering the engine.
the O2 sensor determines how much air is leaving the engine.

the Lambda map takes O2 sensor data to change the way the MAF sensor measures air.

that doesnt mean the MAF is wrong, it could mean the fuel pressure regulator is a bit off, or the injectors are not flowing to spec.

the way it works is the MAF is saying you are sucking in a certain amount of air, the ECU injects the specified amount of fuel, but the O2 sensor is telling the ECU it doesnt match for optimal combustion and adds a point to the lambda map at that RPM and MAF reading for a new fuel injection amount.

the more points on the map, the more accurate it will be, since it interpolates missing points.