Contradiction in Cloudkill Description

Question

I have found what seems to be a contradiction in how the 5th level spell Cloudkill affects creatures of certain HD:

These vapors automatically kill any living creature with 3 or fewer HD (no save). A living creature with 4 to 6 HD is slain unless it succeeds on a Fortitude save (in which case it takes 1d4 points of Constitution damage on your turn each round while in the cloud).

A living creature with 6 or more HD takes 1d4 points of Constitution damage on your turn each round while in the cloud (a successful Fortitude save halves this damage). Holding one's breath doesn't help, but creatures immune to poison are unaffected by the spell.

So for a creature that has 6HD, it is simultaneously under two contrary conditions, either save to stop a death-effect and only take 1d4 constitution damage or only ever get 1d4 con damage and a fortitude save to halve the result of that.

There doesn't seem to be any errata on this despite it being in the core rulebook.

Do I have to houserule a solution or is there an objective ruling on how to deal with such a contradiction?

Answer 1

Let's look at Mythic Cloudkill description

Mythic Cloudkill

As a move action, you can move the cloud 10 feet in any direction. Add your tier to the spell's level and Hit Dice categories to determine its effect on creatures. (For example, at 3rd tier a creature with 6 or fewer Hd gets no save, one with 7–9 HD must save or die, and one with 10 or more HD must save or take 1d4 points of Con damage.)

Extrapolating it back we can see that basic spell stats should be:

• 1-3 HD -- instant death;
• 4-6 HD -- damage or death;
• 7+ HD -- damage;

Answer 2

The difference here is that a creature with 6 HD must save or die on the first round, then it will take another 1d4 CON damage (fort for half) for the rest of the spell's duration.

While a creature with 7+ won't die if it fail it's first save, it will just take 1d4 CON damage (also with a fort for half).

A creature with 4 or 5 HD will save or die on the first round, then it will take 1d4 CON damage with no save to reduce to half.

A creature with 6 HD is simply under two different conditions that should both apply.