Probably just a glitch, its a critical hit and you didn't notice, or your defense was lowered. What move did you use while Snorlax used headbutt?
Amnesia in Gen I only increased the special stat, which only affected Fire, Water, Grass, Electric, Psychic, Ice, and Dragon moves. Headbutt was a Normal type move even back in Gen I, so it couldn't have been affected by it. There can be no other explanation other than the above unless you say what move you used.