I thought armorys wallet is deterministic and you only need to backup it once - no matter how many new addresses you generate afterwards. Isn't it like that?
Correct it is deterministic, but a keypool is pre-generated addresses that can be used. It saves time, and has nothing to do with backup unless you change that number, then you may miss addresses when you restore, but changing that value will return those addresses.
If you used more and restore, it won't think to generate them YET. It CAN, you just tell it to.