A unit does not receive Combat Result points for extra ranks as long as it is disrupted.
A unit is disrupted if an enemy is attacking it in the flank or rear, and that enemy unit has two or more ranks of at least five models. Smaller units are assumed to have insufficient mass to cause disruption.
Sometimes, an enemy unit will begin the round of close combat with enough ranks to cause disruption, but takes enough casualties so that it no longer has two or more ranks of five or more models. In this case, it can no longer disrupt the unit it is fighting and the extra ranks are counted as normal.