Is the fact that the DCs for levels 12 and 13 are one less than you expect a typo or a deliberate choice?
Hehe, when I first created the table I had to do a double take as well. It seems strange that the DC remains the same for so long doesn't it.
The reason is that the levels right before, the party's win rate is just a little low, but not so much that the DC should be lower. And towards the end of that segment, the party's win rate is just a little high, but not high enough to warrant a +1 DC. This is the area I will have to look at hard in playtesting, as its a potential trouble spot.