Inline fuel flow gauges are almost invariably wildly innacurate - 11% out I'd say is actually pretty good.
Use the manual, plus safety factors. Anything in the cockpit of a light aeroplane can only be regarded as indicative
Also do bear in mind that just because the G1000 is pretty advances, it doesn't change the fact that the sender in the fuel line is probably 30 year old technology and cost a couple of dollars.
G