A family of Microsoft word processing software products for creating web, email, and print documents.
Apparently someone thought it would make sense to use a hyphen (like a minus sign) to contrast to Ctrl++ for Superscript. But you can reassign these shortcuts. Go to File | Options | Customize Ribbon and click on Customize... beside "Keyboard shortcuts." In the dialog, select Common Symbols and scroll to Nonbreaking Hyphen. Press the desired keys (you'll see the shortcut is currently assigned to Subscript) and click Assign.
Note that the display above is misleading. When you actually press Ctrl+Shift+-, Word displays Ctrl+Shift+_; if it displayed Ctrl+_, this would be reasonable, and that is what it actually does in Word 2003. Apparently the change was introduced with the Ribbon (I can no longer test in Word 2007, but Word 2010 uses the new display).
You'll then have to go to Home Tab | Subscript to reassign the old shortcut, which has been reassigned to ViewZoomIn. I'm not sure who thought we needed these new zooming shortcuts, but, like the reassigned ones for Copy/PasteFormat, the change has been unpopular.