Depending on your site setup, a user may navigate between multiple subdomains while on your site. For example, they may start with blog.xyz.com, then go to www.xyz.com, and so on.
Google Analytics has a Hostname (subdomain) report located at Audience > Technology > Network > select the Hostname tab.
In this report, the sum of sessions by hostname equals the total sum of sessions. So what happens if someone navigates between hostnames?
In Google Analytics, the Hostname report only shows the FIRST hostname that was recorded. In the example above, the user who landed on blog.xyz.com then navigates to www.xyz.com, will show up as Hostname = blog.xyz.com.
Note: this only applies to the Hostname report. Everywhere else, sessions are associated with all the hostnames they passed through, whether it was the first or last. So there's no issue with, say, making a segment on sessions using the conditions "hostname contains blog" AND "hostname contains www"; it will return those sessions that visited both the blog and the main domain.