-지만 is 'but', and two shapes recur here. One is X지만 알고 보면 or 알고 보니 — it looked like X, but once you look into it something else is true, as with the tuna that wasn't tuna. The other is 모르겠지만 알겠습니다 or 알 것 같아 — I can't say why, but I get it. 알다 covers both knowing a fact and getting the point.