Here's a long answer to a short question.
I read elsewhere on this forum (viewtopic.php?f=2&t=4739&sid=f6622f164a ... 30f#p17652
) that the game author apparently updated the game to sort maps based on how long it took an experienced player to solve each of them. But it's not clear if that meant just the original set of maps, all maps, on what difficulty they were played, if it was sorting just within a given page of maps or across an entire set, if it involved maps changing names or not, when the change was made, etc. etc.. My guess is that this re-sorting effort was done long ago, applied only to set 1, that no maps were renamed, and that it was only sorting maps on their own pages.
Here's what I believe about map difficulty based on my personal experience.
I believe set 1 roughly increases in difficulty across map pages. That is, set 1 page 1 is much easier than set 1 page 18. The difficulty may also increase on a given page, but the difference is small enough that I don't notice it.
I believe this is NOT true for set 2. In set 2, the maps are organized by theme -- here are some famous people, here are some maps of real places, here are some snowflake patterns, etc. etc.. The difficulty in set 2 is kind of a crap shoot; most maps on a page are a cakewalk, including set 2 page 18, but a handful of maps here and there are brutally difficult. It's not clear if anyone has ever solved Sauron on the hardest difficulty level, and it's on page 12.
I'm 50/50 on set 3. Set 3 appears to be more autogenerated maps, but this time with lots of spiky trails. I haven't noticed any difficulty changes across pages, but I've solved less than half the maps. I wonder if the game author did something to cause maps to increase in difficulty across pages, such as either (a) running the AI for all maps and sorting them by how long it took for the winning AI player to win then publishing the set with maps in that order, or (b) tweaking configuration parameters as he generates maps to make them harder on later pages. But that's all speculation, and I don't have enough experience with set 3 to have a strong opinion.
Note also that there's a big difference between "days to win" vs. "how hard is it to find any solution at all". Almost all maps can be won with decent strategy, but there are several maps in set 2 in particular that I was not able to beat via my normal strategy, but I was able to beat by brute forcing different weird starting moves and finally finding a set of opening moves that caused AI players in different parts of the map to behave differently, and once I finally found a toehold I was able to revert to normal strategy and win. But some of those are small maps so "days to win" would not be a good measure of how painful it was to find any solution at all.