The Internet of criminal things

the accusation by the EPA is that there are multiple profiles in the ECU and that the car detects that it's being tested by the EPA and switches to one that pollutes less than the ones that are used for normal driving.

The EPA test profile is very strict and not something that will exactly match any on-the-road test. It was people doing on-the-road emissions tests and looking at differences that raised the concerns and started the investigation.

how close the EPA dyno test is to the on-the-road test that showed issues is part of what ends up confusing the issue (see the samsung TV power consumption issue), it all depends on what the exact triggers are to change the fuel profiles are and what the justification and logic for them are.