... the most [enthusiastic] prototype players who played it often liked a few turns more.
Interesting. I wasn't a prototype player, but I'd definitely consider myself to be an enthusiastic player. In the couple of 2-player games I've played so far, I too thought the length was a bit too short (sadly, for me, only 2 players will probably be how I'll mostly play). That's why I thought maybe the game was meant to scale the way I had speculated. If you "like that it is a bit different with different numbers of players and it makes the playtime a bit more equal for different player numbers" I would say that the total game playtime would be even more equal
among the different player counts if the number of turns per player per year was more evenly-scaled, as I'd mentioned.
To wit ...
The total turns (which approximates the game length) for all players in the published rules vs. the "evenly-scaled turns" is thus:
published vs. evenly-scaled
2 players - (9+6+6)*2 = 42 (9+9+9)*2 = 54
3 players - (8+6+6)*3 = 60 (8+8+8)*3 = 72
4 players - (7+6+6)*4 = 76 (7+7+7)*4 = 84
5 players - (6+6+6)*5 = 90 (6+6+6)*5 = 90
As you can see the playtime is "more equal" with the turns on the right.
Anyway, whatever decisions went into coming up with the number of turns, the game is brilliant, fun, and the artwork outstanding!