Indestructible Units

My opponent in a game has an anti-tank unit and an infantry unit in three cities each. They are all at 63% and no matter what is thrown at them they are always at 63%. I just watched another player attack his units with two medium tanks, two light tanks and five infantry units and his units were all killed in less than 15 minutes but the first player’s the anti-tank and infantry is still at 63%. How is this happening?