I recently was about to attack a castle containing some heroes.
I send in a sacrificial scout as people often do, in order to check if I could take on the defenders.
In the following battle, the defenders side is shown to have +70 ambush.
As my true attack included a ranger with 70 group ward I thus felt safe placing my barbarian to the front, trusting he wouldn't be assasinated. He also had items taking his personal ward even further above 70. (in hindsight I realize it was still a gamble since once he got to the assassin hero, he'd face potentially a lot of ambush above 70 due to the assassins personal ambush skill, but that's not important in this example)
I attacked, and my Barbarian is promptly ambushed on the second enemy, a mage! A troop with no innate ambush. The defenders is now shown to have 21% ambush, despite my 70 ward.
How come the first attack showed only 70% ambush, when the true value for regular troops was apparently 91%?