It's true that a in a 4 v 2 battle, where you start with 3 dice and roll against 2 until one side or the other wins, the attacker should ultimately win the war 65.59% of the time.
However, what your dice stats are showing is only the result of the first roll (the one roll where you actually have 3 dice against 2). On that roll, you could win 2, win 1 lose 1, or lose 2. On that roll, your dice stats are showing that you kill an army 54.2% of the time and lose one 45.8% of the time.
That sounds roughly correct, because on a 3 v 2 battle, you have roughly 37.2% chance of killing 2, 33.6% chance of killing 1, 29.2% chance of losing 2. If you run the math on that, you get an expected kill rate of 53.8%, so your average assault rate is very close to the expected outcome for a single 3 v 2 roll.
Assault odds are accounting for the fact that after that first roll, there is a chance you could still win the war with a 3 v 1 or even a slim chance you could come back and win a 1 v 2. So you start from your 37.2% chance of winning 2 outright, and add to that your chance of wining 1 on the first roll and ultimately winning the fight, plus the slim chance of losing 2 on the first roll but still winning the fight, and you get to 65.59%, which is what assault odds is showing.
That's the difference... hope that helped.