Presumably the Vauxhall mentioned would provide exactly the measured MPG when driven at the correct speed by any owner in the real world - hence the 'flat spot' noticed.
The defeat concept you're referring to (presumably VW's) was set to only function during a test, and not in the real world (AIUI).
VW's defeat concept was set to function only at the exact RPM's used during the test. If a real-world customer were to use those RPM's and conditions, it would activate too. In fact a CCC talk showed that 5% of regular driving activated it.
Perhaps about 5% of regular vauxhall driving was in those flat spots...
