Notice that they had from 5 to 45 times the LIMIT (which is a lot!).
The lower/higher numbers were due to city/highway mode, I think. (I assume the city numbers are the higher ones?)
The variation in the low and high figures themselves was due to the different vehicles tested.
I think, as someone mentioned, and as the news noted, the code is actually covered by the DCMA (it would be nice to find a cite).
It wasn't so much that VW /couldn't/ explain, but that they wouldn't explain it. They only admitted guilt when both CARB and EPA said they would not certify 2016 diesels because they couldn't be certain of the manufacturer's own certification process.
Only then, when VW knew their stock price would take the hit, did VW finally confess. And even then, they didn't confess to the fact that it's not half a million vehicles, but more than twenty times that number!