[mwebb]

on your system
the primary Engine load sensor is the MAF sensor

when there is an air leak the MAF sensor Under reports air flow , so the ECM provides IPW injector pulse on time to provide the correct fuel for the amount of air that it knows about .... blah blah , replay the fuel trim correction thing here

moving along -
now the ECM has been Looking up the indicated MAF value on a table
to help provide other values
Like IGNITION timing , since the ECM sees very light load , IGNITION timing has been adjusted to match what a light load would call for
so
IGNITION TIMING is more advanced than it would normally be for the ACTUAL load
and so on , the ECM is now providing commands based on a much lighter load and those commands are NOT affected by the fuel trim correction from the 02 sensor

so basically
the ECM is reading from the tables that provide commands based on light er engine load - and the ECM provides the commanded values from the tables for the lighter loads
not just Injector Pulse Width
which explains the difference in your FE

sorta
you may have a problem in your system as well , can not tell without seeing some of your
Scan Data at
idle
2k rpm
cruise
WOT

with and without
the self induced air leak - your condition should be repeatable by others with the same system - IF they can duplicate ALL of your conditions

i am assuming the self induced air leak is NOT the purge line from the purge valve on your EVAP system , if it is , it may be the problem is there and your system has a stuck open or leaking purge valve .

if you are near southern NY
i can log that scan data with AE and or Escan on MR laptop