PMI was used for measuring association, owing to its convenience and similarity to co-dispersion, but it should be noted that the specific formulation of the association measure is more-or-less irrelevant in the present context, where we are using relative association levels between a real and random corpus as a proxy for how much structural information is captured from the corpus.
put, the figures show the percentage of times a given association score or lower was measured between word types in a corpus which is known to be devoid of any actual syntagmatic association.