[Graeme Dice]

A longdead starts with 5 hitpoints, so the ones you clicked on were likely the ones that hadn't yet been hit. There are also going to be three longdead in most squares, since they are size 2, so you might have clicked on the one that wasn't damaged out of the two others that were. You can select the other units in the same grid square by clicking on their picture at the bottom of the screen once you've clicked on the square on the battlefield.