When do you check the oil, if you check it when the engine is cold and fill it up then check it when the engine is warm it will show up as low on the dipstick, if you then fill it when its warm when its cold it will show over full.
BTW due to bad oil return in rb's it is reccomended to over fill the engine a little if you drive it hard, that way it reduces the risk of emptying the sump and continuous high revs.