The reason i know is I've made the error a couple of times of checking the oil level on a cold engine, thinking "Wow, that's really low, off the bottom of the dipstick." and topping it up, not realising it had wet-sumped. As soon as you run it, the scavenge pump emptys the crankcase and the tank is then overfilled by half a pint or so.
After a few miles, you get a smell of burning oil and a slithering sensation from the rear wheel as your bike renews it's rear wheel rusproofing..