You can calibrate them to be closer, but there is always a little error. I use mine as a fuel level gauge, as does my dad, and the error that's there is good enough for me. Mine usually reads a little high for flow, so low for gas, and that gives me a small margin of error. My dad's is the same.
But think about the variables you can't measure and can't control. Variances when filling the tank... did it overflow? Did you deduct the exact amount of gas that spilt from what you pumped when you calculated the accuracy? How long has it been since you filled up last? How much evaporated out of the tank in that time, and how much evaporated out of the bowls? I know on both my boat and my dad's, if it sits for more than 2 weeks, the bowls are dry enough that you have to crank it until they're full before it will fire. Did you fill it to the exact same level this time as you did last time?
What about the generator? Did you run your generator, and do you have a fuel flow gauge on that? My dad doesn't... but so far when he's used his generator it hasn't exceeded the "error" in the gauge on the two main engines so his remaining fuel calculation can neglect it.
Since they say you should never use the last 10% in your tank, I think reading 10% extra flow is a good thing, myself!
