All enemies (and I believe this is actually Entities, not just hostile mobs) have an invulnerability period after you strike them.

 

That is when they turn red and only lasts for a split second (for the most part).

 

You can see it when you are striking a cow or the like and while its harder to see on things like a blaze, you can swing your sword to check. A blaze I believe takes three hits with an Iron sword to kill it.

 

If you swing your sword fast you will be able to hit it I believe 4-5 times before it dies as only three of the hits will actually register and do damage.

 

Please note that this is regardless of how fast you swing, and in fact the faster you swing the more wasted uses of your weapon will occur so if you are buffed by a potion, beacon or whatever else may be available to you, then watch your attacks a bit more carefully.

 

(It could have been he just had better ping than everyone else and his hits were registering first, I made an autoclicker macro with my Razer Naga set to a click per 5 milliseconds and still could not do more damage because of faster swings.)