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Why does fat have lower attenuation on CT than water?

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Because fat has fewer atoms per cm, it has a lower linear attenuation coefficient. In other words, because fat is less dense, there is less chance of an incident x-ray beam being absorbed or scattered.

What is the difference between the terms "hypodense" and "hypoattenuating"?

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Although used synonymously for all practical purposes, there is a subtle difference between the two terms. The linear attenuation coefficient (mu) is made up of two parts 1) density (atoms per cm) and 2) a proportionality constant which expresses the likelihood of an atom stopping an x-ray beam. For some materials (such as bone) the material is not only more dense, but the atoms are larger and have a higher likelihood of stopping an x-ray beam, therefore "hypoattenuating" is a more inclusive term.