There are "Normal" Zombies, "Tough" Zombies, and "Special" Zombies. They do increasing amounts of damage, have increasing amounts of HP, and sometimes armor.
Each one of those has a "Regular" version, a "Feral" version (Fast and Aggressive), and a "Radiated" version (Regenerates HP). Again they do increasing amounts of damage, have increasing amounts of HP, sometimes armor, and special abilities.
Each box of that 9 X 9 matrix gives a different and scaled level of base XP for each zombie kill. Base XP also increases as a player increases the game difficulty. (400Xp for the normal and regular is the default IIRC.
Each increasing level of Zombie means more effort and resources for less XP return. The HP and resources required scale faster than the XP from the kill. It's just the way the XP gain is structured. Very few games rely on a linear progression of experience.
PS The list provided from the wiki does not appear complete or accurate. It does not have the specials and what is quoted, does not highlight the three versions of each of those.