The number ninety-one is odd in the literal sense, and it is odd in the structural sense too.
Thirty Aethyrs holding three Parts apiece would give ninety. The thirtieth Aethyr, TEX, holds four. That extra Part is the whole of the discrepancy, and it is the reason every account of this material has to say ninety-one rather than the tidier ninety.
The received material offers no reason. Nalvage's introduction speaks of ninety-one Princes and Spiritual Governors from the outset, so the number was not arrived at by accident partway through, and Dee compiled the table himself and evidently saw nothing to query. Later writers have proposed explanations, generally numerological: ninety-one is the sum of the integers from one to thirteen, and it is the product of seven and thirteen, both numbers with weight in this system. It is also the numerical value of the Hebrew phrase joining the divine name with Adonai, a correspondence that carries genuine significance in Kabbalah. These are real properties of the number. Whether any of them is the reason is entirely unestablished, and none of them is proposed in the manuscripts.
The anomaly interacts with another feature of the list. TEX is the innermost and lowest of the Aethyrs, nearest to the earth, and it is also the most crowded. A reader inclined to find a system in this can say the material world is the densest; a reader disinclined can observe that this is exactly the kind of pattern that is easy to find after the fact.
What can be said without argument is that the anomaly is original, deliberate as far as the record shows, and unexplained.