Well, the interesting thing is that Grice himself is not very clear about the category Manner, which contains the maxim Be relevant . As far as I understand Grice, it is quite a good theory to explain how successful communication can be achieved, but to understand why we understand something which is not actually said (or, why we know what is implicated), I would check Relevance theory by Sperber and Wilson. In an article which they wrote, it is called On Grice theory of conversaton, they discuss such phenomena, especially why you run into problems when trying to analyze sentences like those described above.
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