These models are not reanimated skeletons like the bulk of a Tomb King's army but bound by powerful magic placed inside their inanimate shells. As a result, they are more resilient than normal Undead and always count as if they are within 12" of the Battle Standard, even if there is no Battle Standard on the table. If they actually are within 12" of the Battle Standard then they will benefit from that as well, losing two less Wounds than they should from lost combats. In addition, Undead Constructs have a 5+ Armour save.