Google has mad a small yet significant change in how it defines internal and external links.
What we previously know before as external links are no longer classified that way. Links coming from subdomains used to be known as external are no longer grouped as such.
Based on its Google Webmaster blog post
Most people think of example.com and www.example.com as the same site these days, so we’re changing it such that now, if you add either example.com or www.example.com as a site, links from both the www and non-www versions of the domain will be categorized as internal links. We’ve also extended this idea to include other subdomains, since many people who own a domain also own its subdomains—so links from cats.example.com or pets.example.com will also be categorized as internal links for www.example.com.
What does it mean to us, you ask. Since the classification will likely lower the number of external links — assuming our site has multiple instances of non-www or subdomain links) and increase internal link count, our site’s link popularity decreases. That’s if the long standing assumption that external links hold more clout than internal ones. Therefore, helps recommended we plan some link building campaigns.