The difficulty of a puzzle is determined by three things:
* The author's estimate of the difficulty
* The judge's estimate of the difficulty
* The average solving time of the puzzle (solving time is used as a proxy of the difficulty).
This means that depending on the solving time, the difficulty of a puzzle can change a little bit. If the estimates of the author and the judge put the difficulty right on the border of a 3 and 4 stars, then the number of stars can also change a bit.
However, note that the puzzles with the same star difficulty are also sorted by difficulty. So currently it is judged as the hardest 3 star puzzle (and before it probably was the easiest 4 star puzzle).