Nothing to do with Windows 7.
An Excel spreadsheet is a single document. When you save, you are not saving a worksheet, you are saving the whole document, not just the worksheet that you happen to be working on, just as in (say) Word, if you click save you are saving the whole document, not just the page you happen to be working on. Because you are working on a single document, it's possible to undo any changes on any page (or worksheet) that you have made in the current session.