A designer sits there with 20-something-year-old eyes, on a 27-inch monitor megacentipixel screen--and not thinking about the surfer, viewing the same content with original-issue-eyes on an iPad.
I'm off to get my eyeglass prescription updated. Viewing websites for the last 15 years has really taken its toll on these original-issue-eyes. Had perfect 20/20 up until a few years ago. If you are sitting behind a computer for most of your adult life, you WILL need eyewear or some other corrective action. Websites are hell on the eyes.