Oily skin doesn't neccessarily mean acne either people.
Acne is formed by bacteria in pores in the skin. That's it. It's a form of bacteria that is found on EVERY SINGLE LIVING ANIMAL WITH SKIN. So why do some people get acne and other don't? The difference is in the pores themselves. Smaller pores and such trap the bacteria under the skin giving it free reign to grow and feed off whatever it finds in there without fear of getting washed away or dying from exposure.
If you have good pores but oily skin, you may not ever get a pimple. If you ahve very dry skin and bad pores you can skill get mucho pimples because the dead skin cells fall into the pores and clog them.
Also, the myth about food... not really a myth. Different foods affect different people in a wide variety of ways. Did you know eating bananas for some people causes mosquitoes to litterally swarm over them, but doesn't do jack crap when another person eats a banana? Just a different bodily reaction. Same thing with stress as stress does effect different people in different ways. Eat enough junk food full of sugar, get's your heart pumping for the over dose of energy, the heart pumping faster causes a spike in body temps, a spike in body temps causes a spike in sweat and oil production. BINGO, recipe for ance. Same thing with stress. Does food and stress cause acne break outs for everyone? Hell no.
Look, I'm 27 and still go acne. On my face it's barely noticeable except a few blackheads if you get close. On my back....well I don't like taking my shirt of much. It's been slowly disapearing over the years.
The only good ways to treat acne, since it's a BACTERIA inside BAD PORES, is A) kill the bacteria B) fix the pores.
A dermatologist does A) by proscribing a regimine treatment of anitbiotics to kill the bacteria. This is not a permanent solution because when you stop taking the antibiotics the bacteria just start multiplying again.B) is accomplished through a skin peel. Usually an acid applied to the skin surgerically to peel of a set amount of layers of skin. This is repeated over and over until the pores heal themselves correctly in a way to not clog as easily. This CAN be a permanent solution to acne but it's not garaunteed.
Washing, your face just gets rid of excess dirt, oil, and dead skin cells which is what the bacteria feeds from. It helps to keep acne break outs down, but doesn't remove old acne nor really stop the formation of new acne. Just slow down the overall process by providing the bacteria with less food to feed and multiply with.