My hypothesis is that there are students out there with just a single grandmother to balance out the 3 grandmothers cases. This leads us to deduce the general Law of Conservation of Grandmothers.

More serious note: I have a hard time believing this from my anectodal evidence - my grandmothers were a couple of loving but otherwise spaced-out individuals who were only vaguely aware that I was in school for something at any point. The timing of my exams was not within their remotest knowledge.