I will show you what I mean. I have taken a screenshot of two different websites with the same image. They are both in the same size window and each website is the same size, both using 60% for the mainwrapper I assume.
You see the screenshot on the left. There is a lot of white blank space there which should only be appearing when the window is bigger. Everything has gone out of align and messed up because the window has gone smaller. The screenshot on the right, notice how the blank space from the edge disappears depending on what size you make the window.