That statistic (one in three families) reminds me of a friend I once had who was a student in a small center-left yeshiva. He told me, “25% of the bochurim in the Bais Medrash wear black hats.” When I questioned him, he said, “Sure! There are 4 bochurim, and I wear a black hat!”
I know hundreds of Litvishe families, and I can think of maybe a dozen girls over age 25 in those families, excluding those that have personal issues that would make them difficult or impossible to marry off in any climate. And three of those dozen are in one family, may Hashem send them some brachos and yeshuos really soon! So I would say that in my unscientific statistical sample, that one in 50 families has a marriagable girl over age 25 that is still looking.