Continuously hitting mobs (both passive and non) does not kill them in normal amount of hits

Killing zombies (and other mobs) requires 15+ hits with diamond sword, while continually attacking. This would only happen when I continually attacked them. Continually attacked meaning, not stopping between hits (continually clicking mouse). I also found this true with other mobs, and with other weapons. For example, I was fighting zombies with a diamond sword, and some of them would not die. I then continued to find out why some of them would not die in the usual 3-5 non-critical hits with a diamond sword. I figured out that if I continually hit them, they would almost always die in 15+ hits.