Slitherine

I started my first game as the Iberii and on turn 19 the Lusitani sent an army to besiege my city of Corduba. Within the city were 8 garrison units: 4 medium infantry and 4 skirmishers all of which were at full health and effectiveness. The siege rating was -2 in favor of the defenders. I had 1 wall which was still unbreached. The combat powers of the armies were 11 for the Lusitani and 36 for my garrison. The garrison army was consuming 4 food per turn and the region had stockpiled 69 food. I had plenty of gold, manpower, and metal to pay my army's upkeep.

For the next turn I decided to just leave the garrison inside the city and the invading army decided to leave the region. But I can't make any sense of what happened when I pressed the end turn button. As the Lusitani army was exiting the region, my garrison took two huge hits: the first reducing its combat rating from 36 to 24 and the next one destroying it altogether. On turn 20 the only indication of what had happened was a message saying that the Lusitani had inflicted "a few" casualties against the defender. And while my army was now gone, the region was still in my hands.

I'm still learning how things work, but my understanding was that a besieged army would take casualties if it ran out of food and during the part where a d10 is rolled and added to the siege rating. With a -2, the highest they could have gotten was 8, which according the manual means a low amount of attrition. I don't know of anything that could have happened that would just totally destroy the entire garrison after a siege lasting only 1 turn. It's even weirder that it seemed to happen in two steps: the first as the enemy army was leaving and the second after it had actually moved out.

What am I missing?

Thanks

the D10 outcome is protected in that if they roll a 10 some attrition and/or a breach will occur regardless of the modified score.

now having said, there have been other reports of slightly odd results when an army moves out of a region, so I'd suspect the second set of damage may have been a bug?

Actually, I think I understand what happened now. The garrison on turn 19 wasn't the army that I'd ordered into the city on turn 18. I'm guessing that army was caught outside the city and destroyed, which would explain why the invading army was so weakened. I must have been careless about autoresolving and didn't notice what happened.

The garrison army that disappeared on turn 19 must be an automatic garrison that you get for free and will despawn when the enemy leaves. What confused me is that it was composed almost exactly like the one I'd lost: 4 med infantry, 4 skirmishers, but after reloading I just noticed the skirmishers were "urban militia" instead of the slingers that I'd recruited.

Thanks