Yes, that makes sense, I was thinking it was based on preponderance of kills, but this makes more sense. And yeah it was pretty nice, you can kinda see the bridge in the background... they refused to cross it, and they had only 1 unit of archers, which were right in the front, so I charged them and killed them all off, for the loss of 1 horseman, then just sat and picked them off with my archers until they ran out of arrows, and had killed about 60% or so... and by that time their morale was so horrible that when I charged again they all routed immediately. And yes the reinforcements came in behind them, and mostly just sat there to drain their morale.
I have had this game for a bit, but only just found the Mongolphobia mod, so I am playing with it on for the first time, is quite nice!
If you've come for a visit, you'd be welcome with fewer, if you've come to take me away, you'll need a lot more.