Armistice Loophole...?

In this article that I just read concerning raised alert levels in South Korea it mentioned that the waters off of Korea's west coast are not covered under the armistice's cease fire agreement. Why in the world would that be the case? You'd think that the ceasefire would apply to all Korean territory including its coastal waters extending out at least until international waters begin (somewhere between 50 and 200 miles I believe). Is there some specific reason that these waters were not included?