Question About The Ford Ecu Program (stock)

[gtfpv]

g'day i'm just wondering if you could educate me please on the ECU charactoristics.
THE BA MKII FPV 5.4 specs state 15.3 litres per 100kms. my GT done exactly that consistantly according to the display . after many hours. when you reset it it is all over the place but each time it gets over 50 hours it is always the same 15.3 . and when you fill up . distance to empty is 428kms . this was totally stock .
after the addition of a catback, and dual intake, and bmc filter . the computor was reading 14.4 ltrs/100 and distance to empty 478kms . for several fills . but after a few weeks/ months . the economy has returned to the origional 15.3 and 428 . according to the screen readouts .

IS THIS charactoristic in technical terms of the ECU. IF SO wouldnt this mean that without an edit , no matter what mods are added performance will return to stock economy wise atleast

it sounds very hi tech and to consistant to be a coincidence . with my vehicle .
perhaps you could clarify the facts if there are any here . cheers . ?