from what i understand it has alot to do with the bond that is formed for the longest possible time- that is, if someone else formed a bond with the child for the 1st few weeks but you have had the chance over the following months to form a more enduring bond it is your longer, stronger bond that impacts upon the child.
Form my own experience (which is when i was told that) I had severe postnatal psychosis and my daughter was 4mths old before I got treatment. She was then in a medical ward for the next 3 months without me, so we never really started to form any sort of bond til she was 8mths old. I am extremely proud to say that today, 4 years later, we have the strongest bond I could ever imagine. In some ways it is stronger than that of my oldest girl. Apparently the lack of a bond at the start (which was for the 1st 8mths!) shouldn't impact on her ever again because of her sense of safety, security and love now. She is a mummy's girls and I won't allow anything to change that!