in my experience there are 2 types of people: ones that can focus with headphones on and ones that need silence. you are part of the lucky ones.

also, as far as interruptions go, acoustic interruptions are only one type of interruption. Your brain is physically wired to pay attention to things in your visual fields - it's physically wired to pay attention to people coming and going, continuously assessing the situation. just because you can function in an open space environment it does not mean that the environment is optimal.