Avatar in comments refers to "Website" field in profile?

I found if I put user's the primary blog uri in the "Website" field of the user profile (which is "http://" by defaut), then the avatar in comments works, if not, the avatar in comments will display the default avatar. it seems avatar plugin gets the site_url from the "Website" field of the profile, not the user's primay blog uri, it seems not reasonable, user must put his primary blog uri to "Website" field to let the avatar works in comments.