It's not uncommon for a piece to break off of a die during use. There are some titanic forces involved - striking pressure is some 150 tons, and they run at least 150 strikes per minute. And when you contemplate the design complexity of a given die, it makes sense that there will be areas on the die where pressure is greater than others during the strike. It's why you see the insides of mint marks and letters break off so often. So, yeah, I'm thinking this is one of those small failures, called a die chip. Larger versions are called "die breaks," although there's some argument in numismatics (no, we can't agree on what the words mean ) that a "die break" ought to be a development of a die crack.
That die crack on the link you shared is only for silver proofs. Though I assume that there may be some on circulation strikes.
