Of course, students have no reason to report deaths of relatives when there are no consequences to missing class. So, deaths most likely only get reported during exams.
I suppose it's possible students care about their grandmothers more than their grandfathers.
[1] Figure 2, http://cnx.org/content/m42935/latest/?collection=col11407/la...
With increasing age it is also more likely that you've already had your grandparents die.