According to Harold McGee, it is because your are starting with cold water and going low and slow. This is a good thing--the scum at the top is protein buildup and particles that will cloud your stock and can give it a funny flavor(and in my personal experience will give it a strange mouth feel). By not agitating the bones, you are allowing the proteins to build their little structures patiently and suspend in the stock, eventually settling on the bottom of the pot.
You know how you strain it afterward, and if you squeeze the ingredients, what comes out looks like sandy beachwater and not stock(that's why it's a big no-no to squeeze the bones and veggies)? Well the scum is all that stuff that gets beat out during aggressive stock-making.
So good on you.