This is an often sighted gripe. But basicly you gave up the ground that platoon was on. It does not matter if you loose 40 guys or no guys. They no longer control the ground they started on. In other words........they got up and left what they where defending and the enemy just walked into their former positions unmolested. But guess what? You can do the same thing to the ai.
And routed does not mean lost. They will sooner or later get back to your lines after taking a tour of the country side (and become active again). Put it yet aanother simple way....infantry controls the ground (#1 job of infantry).......armor takes ground......and everything else just supports.
Other things that can happen. You can have forces in a square at the end of a battle....BUT if the enemy has more force and controls the majority of the little flags your force will be forced to retreat (if theres a place for them to go) OR they get routed. things the air chair general needs to consider. But as with all things in APOS......it works both ways. You can force a superior AI force to get routed or forced to retreat with a little planning.