It's probably an estimate assuming that moisture in seeds, combined with bacteria and molds will eventually lead to their demise if they aren't given the opportunity to germinate. However, I've never really checked packets to see whether the expiry date on some is further into the future than on others. Everything now seems to have an expiry date, so maybe the seed packers are just covering themselves to avoid people complaining about seeds not germinating. Apparently keeping seeds in the fridge prevents them attempting to germinate and then failing and dying when there's no growing medium present, so that may extend the lifespan. Some seeds have a very long lifespan, perhaps hundreds of years. If you have a packet which has gone past the expiry date, all you can really do is sow them and see what happens!