Invincible infantry?

Can anyone explain how, even under the most unusual of rolls in the RNG, 9 armies (morale 88%) would lose to a SINGLE (1) unit (morale 81%). No arty, no fort, just a straight 9 to 1 battle with a morale advantage. HOW does this end up favoring the single, seemingly invincible, unit?

ps- it was a 9-unit stack, not fragmented.