You're forgetting about air intake. Your emissions are composed not only of the fuel you burn, but also of the air you use to burn the fuel. I don't know whether that's enough to get you up to 11 metric tons, it seems sketchy.

Personally I think the whole idea of carbon credits is pure shit. The idea that you can pay somebody else off so you can pollute all you want, doesn't change the fact that you're polluting. It's like dumping mercury into the Missouri River but paying GE to stop putting mercury in lightbulbs. It effects a zero net change in environmental pollution, but it's still rediculous.