Your derailers have nothing to do with it (unless you would have chain skipping). The reason you cannot get off you seat on the road as well as you can on the spin bike is that is is harder. You have to factor in balance, etc. It just takes a while . . . you are not the first to feel this way. The best thing you can do is force yourself to get up a lot when you are out training. Start in a high gear (as in harder gear, lower cadance) as it is easier. Keep doing it until you are good at getting up and down without stopping your pedaling. Then start to do it at higher cadance. It is hard when you are flying on the pedals . . . really hard.