The source of fuel in the crankcase shouldn't be much of a mystery, just think like a gas molecule. After getting mixed with air and pressurized into the the cylinder, the biggest escape route is geting past the piston rings in at least one of the cylinders and then down into the crankcase. The only other avenues of escape are in the valves, which would lead upwards to the camshaft and valve cover area, where mixing with the oil would take you back down to the crankcase.