In some early manuscripts of the book of Revelation, the number of the beast is 616 instead of 666. Moreover, despite the attempts of Aleister Crowley and the like to promote the idea of 777 as a significant number, St John of Patmos would have used something like the Roman numeral system, and not Arabic numerals, and so the 'one-less' property does not apply. Therefore the motivation for the number is either gematria on the name of Nero (the 'n' in the rendering 'Neron Caesar' has the value 50 - the difference between 616 and 666) or else the panalphabetic nature of its Roman-numeral representation: DCLXVI. Obviously 1666 would have been even better for this, but that would have been harder to tie to the gematria.