To offer the benefit of the doubt here, perhaps she refuels once enroute and doesn’t immediately upon arrival, and thus forgets to count the fuel from the second half of the trip? It’s curious that her 37 mpg figure is right about double what you’d expect.
As an aside, I recall a story (possibly from Car Talk) about a guy who would brag to his colleagues about the stellar fuel economy of his Beetle. To mess with him, his colleagues started adding gas to it in between fill-ups. After a while, of course, they dialed it back, and eventually started siphoning gas, and watched as their colleague struggled to explain the change. Maybe you should start siphoning gas from the van and see if she mentions her mileage dropping or continues to lie about it? 