CSS: Why does fixed element NOT start at the top of the page in this example?

[HoodiboY]

In this example, why doesn't the fixed #header (the at the top) start at the top of the page, but in this example it does?The only difference is that i changed the "margin" of #content to "padding" instead (repsectively).So why does this tiny change have a huge impact? How do I put the fixed #header at the top of the page while still using "margin" for the #content?