This has been bugging me for the last few weeks. I'd get a bunch of coins and, since I completed StreetPass Quest II, I'd exchange them for new puzzle pieces. Twice, I've had about 60 coins and both times I got less than 10 new pieces, despite having (much) less than half of the available pieces. I've noticed this before, too, only whilst spending less coins. Is it just me? If I was going to speculate (and I am), I'd say that the odds aren't entirely based on the number of pieces you have. Perhaps it picks an incomplete puzzle at random from your collection, then rolls for a random piece of that puzzle separately? Does anyone have any ideas on how it works?