Experts and BHG readers answer.
A lot of factors can affect the viability of packaged seed. For greatest longevity, they should be stored in cold, dry conditions. (I always keep my seeds refrigerated.) It's certainly possible that seeds held in conditions that are too warm or too damp may lose some viability.
There are also considerable differences in seed vigor and viability from the outset. That is, some seeds may germinate, but produce weak seedlings that may not emerge well. Vigor of the seed should be considered as well as viability. It's always a good idea to purchase seed from a reputable source; one who will stand behind the quality of the product.