Why are there different numbers of puzzles of each grade in the library?

A. The library is designed to hold 1000 puzzles. There are currently eight grades, giving a target of 125 puzzles for each grade. However, puzzles of some grades are harder to find than puzzles of other grades. So, to prevent an almost endless search for the 125 puzzles of the rarest grade, some leeway is allowed in the number of each grade. This leeway is currently 10%, or 12.5. This rounds up to 13, which when added to the 125 allowed, gives 138. So, the net result is that the easier grades to find will probably have 138 puzzles each, whilst the hardest grade to find will have substantially fewer.

Note: from version 3.6 onwards, the above is no longer true. The library will contain 300 puzzles of each grade.