UltraGauge Volvo Problems?
 

So I have been having problems getting my Ultra Gauge to give me reliable numbers for instant and average MPG.
First off, this gauge worked flawlessly on my '99 Saturn after calibrating, but the '04 Volvo V70 I drive now feeds it info with the engine off. If I sit with the key on, my AVG MPG keeps climbing. What's going on here?
I just filled up today and when I got home, my average MPG went up almost 1 MPG in the amount of time the UG took to shut down.