confused about armor protection and natural protection

[onomastikon]

I thought these two stacked, i.e. if your natural protection is 5 (thick skin, barkskin, etc.) and you have an armor with a value of 14, then your protection would be 19.

But I noticed that when for example putting on the Amon Hotep, which increases my natural protection to 30 (like if I cast the Invulnerability spell), all other armor gives me no more protection. I double checked -- the same thing happens when I cast the normal spell.

Anybody know the rule on this? At which point does armor protection no longer add to natural proteciton?

thank you