This is because some organismsbhave adapted to become immune to these poisons.
Most neurotoxins are transferred through the blood stream, and use the blood cells to transfer to the nerves. Pufferfish poison, for example, follows sodium channels in the blood. It can paralyze the lungs because it follows Na- channels, but not the Na+ channels that lead to the heart. This is because humans have indirectly evolved, or evolved for one reason that just so happened to help in another way, to become partially immune.
Another example, bee venom won't kill you if you aren't allergic, however, it can kill some other organisms
