It's because the witdth of your section is only as big as what's in it. In this case this means your wrapper which is set to 960px. Setting the section in percentage only works as percentage of the available screen, so width:100% wouldn't solve this. You should set your section width to a specific number and that would fix the issue.
Edit: Use min-width instead of width and it works even better for when you go bigger than you min-width.