Thanks for following up Josh.
It's true that many of these terms are low volume.
For the most part, these keywords have low traffic either due to actual low # of searches, or because they had low quality scores and have been paused (I'm pretty sure RARELY_SERVED only applies to the former case).
I haven't had time to dig in much more and I can't say that I've found a satisfying explanation yet.
In the meantime, I was able get some help from my account team to help answer my immediate question reg. account performance.
As such, understanding this in detail is not a priority for me anymore.
I have to admit.... it still has me puzzled... maybe I'll be back once I have some more data on it :)
Thanks again for the help.
JK