That's Sure Strike. It's a skill the sniper class has. It has a % chance of triggering equal to the unit's level(the sniper's, not the enemy unit's) and guarantees a hit regardless of your chance to hit. If Innes is level 20, he gets 20% chance to ignore hit chance and hit anyway. Some other classes have skills like this one. Generals have a chance to take no damage on a hit, Wyvern knights have a chance to ignore enemy defense, and Assassins have a chance to outright kill a unit.