Hello,
My understanding of this is when vehicle goes into closed loop emission control, the "cheap"/aftermarket cat has not reached operating temperature therefore the downstream lambda mirrors upstream , continued driving/MOT test procedure will eventually "light" the cat and make it work efficiently to pass the MOTC/give a weaker mixture to downstream lambda.(Im thinking the ceramic will warm up quicker/give even heat distribution
Maybe im wrong but thats how I explain it to my customers.
MARK