I laid mine on it side to install DynaBeads in the rear tire (PITA) and got the same results, it is flowing thru the vent tube I believe and not over flow. I removed the charcoal canister from my rocket (for some strange reason I ended up with a California bike in Michigan) otherwise the it would have caught the spill. Most bikes have a tip over valve that stops fuel to the engine but not many have a valve on the vent. It looked to me as if the leak slowed after it had created sufficient vaccume in the tank. Mine was rather full so it didn't leak much. If you tank was lower it would leak more..
Just my observation... Subject to further debate