HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Once, someone sent me a save file with the message, "The computer wins quick battles in situations where it's impossible to win manually." I first won the battle manually (after several attempts), and then, after several attempts, I managed to make the computer win the quick battle.

The thing is:
the damage of most creatures is not fixed, but rather exists within a range of min - max. The amount of damage a creature deals is determined by a random number generator.
In the situation described above, it turned out that in the quick battle, the creatures of the winning side dealt a sufficient number of near-max or max damage, while when replaying manually, they dealt average or below-average damage.

The ways of heroic randomness are inscrutable.
Even if it seems that some pattern is emerging (the computer always wins in quick battles, while a human loses), the truth is that it's still about randomness.
But I will take a look at the save file...