This is from page 117 of the 2006 S&I guide.
Butt Blocking in 1 and 2 and face tackling in 3 are both tactics which involve driving the face mask, frontal area or top of the helmet directly into an opponent in blocking or tackling respectively. both result in a foul for illegal helmet contact.
If the Rule 2 definition was exactly the same as the S&I would Butt Blocking and Face Tackling be called more often?