I don't buy that the Duke strategy is that good. It'll be interesting to see what the flaws in your bot are that make this happen. Here are a few things it seems it's missing:
1) The duchy player shouldn't grab all the duchies before turning to dukes IF her opponent is rushing only provinces. Exactly when, you have to gauge based on the speed of the opponent's deck, but 6 duchies is a decent rule of thumb (sometimes 5, sometimes 7).
2) The provincing player has some interesting decisions on when to turn for duchies. I'm not sure that ignoring province buys for them is good. Maybe not even Gold buys is very good, depending on the circumstance. On exactly $5, definitely duchy.
3) I really doubt that going for Dukes is going to be very good at all, unless somehow they were heavily invested in the duchies, or it guarantees a win, or there's no way to run a 3rd pile.
4) If there's no way to run a 3rd pile, this is good for the province player. This hardly ever comes up.
5) Province player probably wants to buy estates only when it basically guarantees a win.
6) Duchy player shouldn't pass up dukes for provinces if she has 6+duchies in most cases. Essentially never if it is 7+.
Beyond this, probably the big thing is that almost any set of kingdom cards is going to provide good acceleration that's going to be of significantly more help to a Province player than a Duke player.