Why does an open pinfu get 2 extra fu added to the score?
I wonder is it related in any way by the fact that it is the lowest possible score?


  THE SMALLEST POSSIBLE SCORE Many charts won’t have a value for 20 Fu, 1 Han. That’s because you just can’t make a hand that cheap.
  
  What happens if you make an open Tanyao using Shuntsu only, a Ryanmen wait, and win by Ron? Should be worth 20 Fu, 1 Han right?
  
  Actually, such a hand is called “kui-pinfu” or open pinfu, and will be scored as 30 Fu, 1 Han. Since a kui-pinfu is the only way to get 20 Fu, 1 Han, we can thus say the lowest possible score for a hand is 1000 points.
  
  Unrelated to kui-pinfu is the 30 Fu, 1 Han hand won by Tsumo. Such a hand is called “gomi”, because of the payments (500, 300 = 5, 3 = go, mi), and because “gomi” means “trash”.


Wikipedia has this to say, since a no-points hand must be self drawn, add 1-han yaku of self pick to the hand.


  The reason why there are no scores in the 1 han/20 fu cell is that such a hand is impossible. The only 20-fu hands are the no-points hand (pinfu, 平和) where the winning tile is self-drawn.However, since a no-points hand must be closed, making the win via a self-drawn tile automatically adds 1-han yaku of self pick to the hand. Therefore, a 20-fu, 1-han hand cannot possibly exist.