That is odd?
Is it a difference like 1 mpg- 51mpg vs 52mpg or more?
I wonder why??
Could it have anything to do with the soot filter?
I know the trucks-and guess the cars- have particle filters that REQUIRE raw fuel be spewed into the exhaust-onto the filter-to get the soot hot enough to completely burn away
When first instituted on the trucks in 2007-there was lots of bitching about lost FE??
Now maybe maybe these little turbos produce less soot at 70 mph than 60 mph-so they don't have to waste fuel heating up the soot so it will burn away??
Well-that is my best and only guess??

Is Your wife is just reading the NAV systems mpg now-not doing the calculations at fillips?
Are GMs nav mpg indicators "honest" ? Some are dead on-Prius sure isn't
our Prius consistently LIES- shows at least 2-3 mpg better than calculated-yeah-for 78,000 miles-always 2-3 mpg more than actual-can't possibly be by accident!!(gets great mpg but I would prefer the truth)