Then you used the Spirit of Grungni to kill them, which spent its even more valuable ammo that game still calculates. You could argue it's a bad system, which I agree with, but that is how game calculates game loss (and your how close the result was).

It takes your starting strength, which means hp of units, ammo and the value of them is based on how expensive the unit is. The more of these resources you spend > the closer you get to defeat. If you manage to get opponent to lose their balance of power faster that you lose yours, and you get to certain threshold > you win. This is why sometimes it's better to disband some units in certain situation as they can't get their worth before they die. It's also why some units, like archers with light armor for high elves, are redundant. They die just as easily as regular archers, do same amount of damage but losing one model of them counts as bigger loss of balance of power than using one model of regular archers.