Without a viewport meta tag some mobile devices will render a webpage at a larger size (e.g. 980px), then scale it down to the actual device size (e.g. 320px). This was done because originally web pages were not designed for mobile, so if they had been rendered at the actual device size they would likely have looked all wrong/squashed etc. Instead they were rendered at a more appropriate size for desktop and scaled down, this made them more likely to work.