Once a static expression is created, it never changes. Yesterday's newspaper edition will be the same, even if you look at it a hundred years from now. A "web page" changes all the time. The homepage of CNN.com changes extremely frequently. Yet appearances can be deceiving. What exactly is changing?
It is entirely possible that the index page of a dynamic site, such as a weather site that allows you to specify your location, will never appear the same way twice, not even to the same person. On a truly mundane level, there may at least be a clock on the page that always shows the time the page was accessed. Yet when you reload a page twice, separated by two minutes, and only the clock changes, in some very real sense we want to say that intuitively, the page hasn't changed. The message changes on a moment by moment basis in the most literal sense, but that's not how we think of it.
It is very hard to make a systematic, formalized definition that matches our intuition, though.