The pyramid implies (to me, at least) that the lower layers are absolutely necessary for the next layer up. It appears that Maslow himself wasn't so absolute about it, and referred to the degree with which lower-level needs were fulfilled.

It is arguable that the "absolute" interpretation is not fundamental to either the pyramid or the hierarchy. It's still probably a good first order approximation to get the point across: you're gonna have trouble with upper layers if you have any serious deficiencies in lower layers.