Unfortunately it's hard to answer that question without better understanding your implementation. There are a number of possibilities that could be going on. Here are some things to check:
event12 is purposefully 'serialized,' which means it is set in the admin console or in the tag to only be counted once per visit
event12 is getting called somewhere else. My suggestion for this is to break that metric down by your pageName eVar. This way you can confirm whether e12 is firing anywhere that it shouldn't be. Try breaking the metric down by Custom / Download / Exit Links to see if it is ever firing there, improperly.
The pageName value for that pageview is duplicated in a place where e12 is not firing. If you have one, consider breaking your pageName value down by an eVar that has URL in it. This way you can see what URL(s) align with the pageName, and if e12 is missing on some of those URL's.
Of course, there's a wealth of other reasons that this could be happening from an implementation standpoint, but hopefully this gets you started as you dig into the root cause.