If it is like mine, it only does it if I top it off close to the brim. What occurs on mine, is that I may fill a little after the auto shutoff from the pump to get that last little bit. I do it on the left side first, put the cap on, then go to the right site. Do the same, then when I lean it to the left, the right side is higher level the left and it finds any little place to come out due to the left fill cap being lower than the level of fuel in right.
I can torque the cap down tight, but then I worry of getting it too damn tight. Best thing for me has been to get it straight up and ride it, as well as taking it easy on the topping off.
One thing I have done though that seem to help, is take a Allen Wrench and tighten the plastic threaded insert that is screwed to the metal cap, I think it get a little loose after awhile and allows fuel to seep past the threads of the insert when it is real high and leaning to the left.
Just my 4 cents