Editors of this wiki measure plant damage using "normal damage shots" and plant health using "bites." While these are good approximations of the real thing, that's all they are - approximations. Nothing beats the real thing. This post will attempt to explain that and what really goes on behind the scenes.
In the game, damage and health is measured in a different unit, which I'll just call HP (hitpoint). One pea deals exactly 20 HP worth of damage. Throughout the article, damage will be in this unit. Basic zombies have 270 HP; most plants have 300 HP.
But you'll say: "Wait a minute... If zombies have 270 HP, isn't that 13.5 NDS!? You are WRONG! Basic Zombies only have 180/181/200 (9/9.25/10 peas)!" Nope, I am not wrong. Zombies have 270 HP, this is absolutely correct. However, they reach a critical point once they lose 2/3 of their HP. This is a point past which any attack will decapitate (but not kill) the zombie.
A decapitated zombie is not truly dead as far as the game is concerned; however they will rapidly and automatically lose HP until they are truly dead. This is why plants still target decapitated zombies and their bodies still absorb projectiles for about a second. Some zombies (mainly Gargantuars and FF machined zombies) have a critical point of zero. This why their headless bodies absorb no extra shots. This is why Bug Bot Imp appears to have more health than other Imps.
Furthermore, zombie attacks (mostly biting) are measured in DPS (damage per second) rather than "bites." Almost all zombies have 100 DPS; that is it will eat a normal plant in 3 seconds, a Wall-nut (4000 HP) in 40 seconds, etc. Two zombies munching on a plant simply double the DPS, ie: two zombies will eat a full-health Wall-nut in 20 seconds. This greatly simplifies calculations and frees you from counting how many bites a plant has received.
For zombies, I also included a "Perceived HP" stat, which is how much HP they appear to have (ie: how much damage they take before they get to the point where they lose their heads).