Ok,
Look at this fuel map.



Ive highlighted the lines im talking about in blue so you cant miss em
Next to the lines, in the map sensor pressure boxes ive written in a boost pressure example.

So:
Top line will be hit at 28psi and above. so at 28psi we are on the top line and at 50psi we are STILL on the top line. Ok?

Good.
Now if we are running 20psi we will be fuelling from the 2nd line.

SO:
If the top line is mapped to fuel perfectly well at 50psi and we drop the boost to 29psi we will be very rich. But if we drop to 22psi we will be fine.

Does that help?

**Disclaimer**
I know damn well the pressure examples are wrong, before any smart arse decides to try and correct me... They are wrong on purpose