Furthermore, more conscientious students are more likely to grin and bear it, thus giving the A students a lower rate of reporting.
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.