Sweet! I didn't understand how something that was sterile when packaged and remains in a sterile environment could then mysteriously lose its sterility???
The same way that a helium balloon goes flat in just a couple of days -- the helium molecules are smaller than the elastomer that the balloon is made of, so they leak out.
Medical packaging, no matter how intact it may appear, will still allow transfer of microscopic organisms at some rate. Granted this process may take several years before any appreciable amount of foreign matter has entered the packaging, but eventually it will reach the point that the manufacturer is no longer willing to claim that the item is sterile.
Still though, an emergency dressing need not be sterile, just clean.
Remember, you're not providing definitive care and sealing up the wound forever, you're just trying to get to hemostasis until your patient gets to a doctor. The first thing they'll have to do is remove all your hard work then irrigate, clean, debride, and actually treat the injury. The fact that you didn't have a sterile dressing on there is of little consequence, as long as it's not covered in dirt and poop.
People often fail to realize that whatever is was that created the wound in the first place most likely wasn't sterile, so as long as you don't make the situation worse, you're doing no harm.