Why was Rasputin claimed un killable?

You've probably heard the condensed version of the tale; he was poisoned, shot, beaten, and finally drowned at a dinner party in his honour. Though, lest you get the wrong idea, this was not just how they partied in Russia at the time. No, it was Rasputin's benefactors - who had grown tired of his meddling in political affairs - who decided to put an end to his influence.

Rasputin's friend (or so he thought) Felix Yusopov invited the mystic to a late-night social gathering. Now, to spare you the minutia of detail, much of which is in question, what is known of the meat in this story is that Rasputin had been served wine and pastries laced with cyanide at this soiree. Most accounts agree that he reluctantly imbibed copious amounts of the poisoned wine, though he may or may not have eaten the tainted pastries. But for whatever reason, he did not succumb to the toxin, except by way of becoming quite drunk.