loose gas cap v mpg display

[burp]

I forgot to put the gas cap on after a fillup once and drove about 50mi. I also noticed that my mpg went from 32[tight cas cap] to 50[loose]! Whaaaat? is that real? I put it back on tight, and drove 50m more. MPG went back down to 32. Then went another distance with it slightly loose, and got 48mpg on the display. So it is repeatable, but what is going on?
I think that the display is being fooled to give a high reading. The emissions system keeps a slight vacuum on the gas tank. When it does this with the cap off/loose some fuel in the form of vapor or liquid enters the engine and bypasses the fuel injectors. Since the fuel injectors are used to calculate the amount of fuel used, the denominator of miles/gallon is reduced and the displayed MPG is high.
When mpg is checked by [miles driven] / [gallons of gas pumped] it stays close to 32 previously observed.

Has anyone else observed this affect?

24 hyundai tucson hybrid