It's because you put a 1% left and right margin on your #header. In this case, the percentage is relative to the browser width, so when you make the browser smaller, the value of the margin is recalculated and - as a result - smaller, making the div move more to the left. I did a test in Dreamweaver here; I removed the margin and it stays put. Replace the percent with a value in pixels.

And please don't use tables. They are not necessary for layouts, except for tabular data. They create bloated code and accessibility issues for disabled people using screen readers.