It's like J08M3 said.
MAP sensor (Manifold Absolute Pressure)
MAF sensor (Mass AirFlow)
The S65 V8 M3 uses a MAP sensor.
If you wanna learn more on these sensors readup on them on Wikipedia.
Map sensors don't see changes in engine volumetric efficiency as well as MAF.
Since BMW tuned the DME according to it's factory tested % of engine volumetric efficiency, with required values to make there performance requirements.
Now adding a few extra % of volumetric efficiency on a engine from a exhaust and air filter. The DME doesn't see them as well with a MAP sensor and cannot adjust the fuel for optimum
But the engine has safeties built in so you don't damage the motor from leaning out.
By using the integrated 8 knock sensors and 4 O2 sensors.
Now this is what i know, but i don't know all the DME functions and tune. As i'm no tuner.
Maybe tuners on here will clear some of this up.