All in all it's because the warscore from battles calculations is extremely nonsensical.Total battle size is just as important as relative aemies in my observations. Smashing a single 9k army is worth a lot more then smashing 3 3k armies. So smashing 15k causing a fatal blow is not that surprising to me.
For the most part the current warscore from battles weight seems to work fine, but it will throw curveballs like that once in a while.