People who tend to have had secure attachments early in life only tend to have higher well-being later in life because there are other factors influencing the matter. Other factors may be the condition of family life for the child and the character he/she had the secure attachment to (The adult may not have been a very good/kind person, but still loved the child to be the caregiver of secure attachment). Also, I think it is important to see whether the attachment lasted for a long time, as other events may have ruined the attachment or changed it, ultimately causing harm for the child. Perhaps for cases where the secure attachment lasted throughout most of the child's early life, a very high proportion of those children do have higher well-being.
