I can imagine this as not totally different from software dev and QA. Many times the development team puts in some feature that doesn't have any visible output for the testers to test - rather it is some internal change or improvement.

The QA department, then responsible for testing this new "feature" will most likely go off of a script that was sent to them by the developers to verify the old and new behavior, going on some metric that perhaps they don't really understand.

In some cases, the QA may actually have to rely on a tool actually built by the developers to test the feature. So in some cases the QA, not being software developers and not responsible for actually reviewing the code, will have to somewhat go on faith that the scripts and tools provided by the developers are accurate.

I'm not saying that means the engineers were responsible at VW, but it does explain how something could slip through the checks-and-balances that have been set up.