Each level of alien armor has a certain damage absorption that it has. For example, lev 0 carapace would completely absorb 5% damage, lev 1 carapace would absorb 10-15%, and so on. Then, the damage is distributed between the health and armor. At level 0, like in the manual, 70% of the damage goes to health while the other 30% of damage goes to armor. The damage that goes to armor is halved. When there is no armor, the health would take the full amount of damage (after subtracting the percentage the carapace initially blocks). At level 3 carapace, the damage absorbed is around 30-35% and would explain why a lev 3 carapace skulk can take around twice the hits a level 0 skulk can.
Marine armor has no initial damage absorbance. The full damage is taken and the damage is distributed between the armor and health, with damage to armor being halved. This would explain why whoever did the balancing of the game has increased amounts of armor for each level for marines and none except for lev 1 carapace for aliens. This is also why welding armor is important for marines.
The manual's levels for damage absorption are accurate, but they have no mention of how much damage the aliens block completely with each increasing carapace level. I think it's roughly like this:
lev 0: 5%
lev 1: 15%
lev 2: 25%
lev 3: 35%
That's just an approximation. Could someone give harder figures?
Before someone spouts off that I'm wrong, actually test this stuff before jumping to conclusions...