maybe because they're part of a "$foo High School class of '99" group/page/whatever with some friends, or perhaps both parents and grandparents are linked as fr

maybe because they're part of a "$foo High School class of '99" group/page/whatever with some friends, or perhaps both parents and grandparents are linked as friends, making it pretty easy to figure out mother's pre-marriage surname? It's not just the explicit facts you provide, a significant part of the deeper value is from the linkages between them.