Another aspect of this is 'software' updating - regardless of the declared or undeclared intention.
That, btw, also applies to Emissions standards.
Toyota IMO is 'manipulating' the system every way possible - when needed on these two and some of the other noted above.
try to max benefit and in that
sense 'manipulate' the system - it gets down to the details and results more than the concept of 'manipulation' ie how rather than why - and is it a 'legal' 'method' and 'result'.
If you add up all the imperfectly known over delivery and under delivery in the real world, my gut is that GM product ends up the most positive ie you end up with the best distribution of actual fuel economy performance versus EPA - that would be in regards to 'corrected' and 'uncorrected'.
Manipulation and 'legal' are sometimes discussed as the same 'thing' and sometimes they're not - it is
a separate question - and at this point, worth investigation - on at least some.
There is also the 'rounding errors' present thru out the whole system.
A savy competitor that comes right up to the line and doesn't cross it is just that , a savy competitor, - who is using one kind of 'strategy' - although that will mean in the real world somebody may have a better under promise and over deliver performance or really, 'ratio'.
Regardless, there are factors in all this outside of OEM control that effect the result.
We are also now beginning to see even in the regular gassers when looked at by themselves ie w/o diesels, hybrids etc - that they now decay or surge ahead in a wider range of numbers - both individually and as a group.