Just wondering what the reason is that it is on boost that it should read wrong?

I mean how does the sensor "know" that the car is on boost.
How about a high reving NA engine - would that read correct?
does it make a difference if the boost is 1 bar or 2.5 bar?
what about the size of the turbo - does that influence the incorrect reading?
and what if the boost comes in lower revs compared to higher revs - is that still the same.

I am just mildly sceptical to the statement: "it reads 0.5 AFR rich on boost"

Is that just a very simplified explanation, or is there no more to it?