Fuel Used Problem

I currently have two E127s interfaced with my Yamaha SHO 250 and receiving engine data properly. I set the fuel tank size at 50 gallon and am getting, it appears a proper fuel flow reading but when the boat sits for a while with power off and I turn the units back on all the fuel is showing as consumed even if I only consumed 2 gallons on my last trip.

Thoughts?

Thanks in advance.