While acne often makes its presence very known above the skin, the reason it leaves scarring behind actually has to do with what's happening underneath the skin. During a breakout, inflammation is happening underneath your skin. This inflammation causes trauma to skin tissue — leading to scarring. Clogged pores are usually caused by excess oil, dirt, and bacteria becoming trapped inside a pore and creating a pimple or blackhead or whitehead; unlike milia, they're extractable and if you squeeze, all that gunk will come out of it. There may be oil trapped in milia as well (along with the keratin build-up), but it's trapped much deeper and covered by layers of dead skin. Certain people have a genetic predisposition to form milia. Usually these people have very dehydrated complexion and narrow pores which makes it hard for oil to exit and easier to be trapped inside. Milia usually forms and comes back in the same areas where people have overactive sebum glands. Another reason for milia formation is silicon based heavy skin concealers and foundations.