In Planning Poker, cards usually contain numbers of the Fibonacci sequence, which is 0,1,1,2,3,5,8,13,21,34,55 etc. However, you can see on the Wikipedia page (and this has been confirmed to me by people that work at several positions where Planning Poker is applied) in some editions the cards stray away from Fibonacci sequence after 13. They lower 21 to 20 and then continue with 40 and 100. Is there some rationale on why these values have been changed, specifically 21 to 20? (Also note that some other cards are added, such as ? and 1/2, but these are easier for me to understand, compared to the 21 - 20 shift.)