Surrender Accepted!?

[Chaotix]

I am blown away by this development. Since ETW was released, I have not heard of a single settlement actually surrendering- it seems every time, no matter how small the garrison is, they always fight to the death.

And yet, here it is, captured not 10 minutes ago.

Spoiler Alert, click show to read: 

Austrian campaign, M/M. I attacked Sofia, Bulgaria, held by the Ottomans, with a force of around 13-14 units. The enemy force amounted to about 9 units, half of which were armed civilians.

Now the question is, why would the AI surrender where the forces are of at least comparable size, yet not when they are defending against a full stack with two units of armed civilians?

EDIT: I'm not sure what's wrong with the pic, but as long as it's visible, I don't care much.