The ARM forums / discussions spread text out to fill the whole browser window on modern widescreen displays. This gives unreadably wide lines that are hard to scan. This style digs into the page and selectively narrows the columns of text inside posts to be closer to the ideal for scanning and readability. It does this while leaving alone the gross structure of the page to continue to fill the window and while not modifying any other aspect of the site style such as color or font choice and size.