It kind of makes sense. We don't go out into the world thinking "ok, time to meet more my moms!". We can't treat anyone else like our mom, and vice versa. The mother-child relationship is probably the most important and influential for most people, but it's a complete anomaly. Friends are not an anomaly, their pretty typical, and since Mom and Dad are not friends, a young child has learned very little firsthand about friendship until it happens. Then they have some experiences with those friends and those experiences become better predictors of future attachment style than the dynamic with the mother. This makes sense because in the realm of friends, these are their formative experiences. Mom may have had an impact (and does according to the study), but the bigger influence on how you deal with future friends is in how you experienced your formative friends.