Well that happens a lot, specially in boss fights. I am not sure if it was coded that way, but whenever a monster is on the brink of defeat, it suffers a drastic boost in either its strength, evasion, and or defenses. It does happens, it can't be avoided unless your strength is drastically much greater than his defenses. Also, it depends on the weapon you are using, the body part you are attacking and the equipment in each bodypart.