If twenty twelve is responsive, why is there a set column width ?
Quick Specs (all measurements in pixels)
The main column width is 625 except in the full-width layout where it’s 960; sidebar width is 250.
Featured image width is 624 (height is flexible)—these images are displayed on displayed on “standard” posts; posts without a specific format such as image, aside, link, or quote.
The suggested header image dimensions are 960 by 250 (width, height). However, both are flexible so you can upload a smaller cropped image if you wish. The width is limited to 2000.