In VW's case there was some reason to believe that at first. There are reasons to test an engine in the lab without emissions controls, and so it is believable that a lazy engineer could put in code for the lab that also triggered in real world conditions.

Note that the above is only believable before the rest of the investigation. Proper investigation proved the story wrong, but it is just possible enough to believe it could be true if the proper investigation hod come up differently.