That confused me as well-so much so that I even looked up a forum dedicated to the series to see their thoughts.
I didn't like that fact because I liked the idea of Tyrion killing the man we always knew-the polar opposite of Bobby Baratheon, disciplined, cold, etc.-but now it just seems like he killed some random chap we never really knew in the series. I know that Varys hinted that he created those tunnels for an unnamed previous HOTK to get his wenches, but that was too vague.
Another weak point, agreed. JRR Tolkien would never have left such a loose end.
In other news, good luck getting through the fourth book.
Cersei has about 500 chapters and Martin's writing just completely deteriorates when writing from her perspective.
No "seed" is left "unplanted." 
