Expiration is when your computer plans to delete its stored version to force a refresh next time you visit, but at the same time you probably (I know I do...) have refreshed the page manually (refresh button ftw!), and some browsers even have a setting to auto refresh each time you go back to visit the site again (so there the exp. could move forward without a user refresh). Thus the file is now newer, and the expiration date moves forward. It has nothing to do with how new the page information is.
Maybe through explaining this stuff it will stop coming up repeatedly. Oh, and in case someone decides to nit pick my previous post, there are actually more things than those two going on each time you visit a page... I just cut to the two that mattered at the moment. Yay college Network Programming class!
Okay, I'll tell you what this means. Each time you visit a website your computer saves what is there. Each time you go back the computer does 2 things. First, it checks if the currently stored version is too old (browsers vary on this) if it is, then it loads a new copy. Second, if the stored copy is not too old, it sends a message to the server asking if a newer copy exists, if so, it downloads the new version, if not, you get the cached (saved) version. The screen you have shown only says that your computer last downloaded that page (for whatever reason) today. It does not necessarily have to do with whether the page has been updated.
EDIT: Edited out angryness... Didn't mean to sound angry, but when I reread it I thought it kind of did.