My guess is that any expiration date on a box is the result of opinions from the companies legal department
You will find an "expiration date" on about every product that contains a chemical compound. In food its usually a "best used by" date.
It doesn't mean the product is useless after that date, all it means is, after that date, the maker does not guarantee it is usable.
This is something you can easily see with paints, solvents, glues, etc. If you talk to the maker, they will tell you that if it still mixes up properly isn't dried up, crystalized, etc., it should still work, BUT they won't guarantee it's still good, nor will they stand behind it if it isn't.
I once worked on a project where the end product was required to last 40 years. Because it was required to last 40 years, it was engineered and constructed to last
300 years. That way, the people required to sign off on it were confident it would last 40 years (and also that in 40 years most of them would be retired or dead
) and so they certified it.
Putting a 25year shelf life sticker on a box of ammo is simply something that eases the mind of purchasers, and provides a bit of legal protection for the maker. If the maker says its good for 25 years, then you can be confident that it will be good considerably longer than that, but how much longer, no one can, or will say.