When a secondary site is created in a client account, the system automatically prepends the primary site name to the subsite name. When the user saves 'subsite' it is actually saved as 'primary site - subsite'. If the user edits a subsite name, they can delete the primary site name and it will not be re-added. If the primary site name is edited, that portion of subsite names is not updated to match.
This system was devised to make it easier to associate subsites with their primary site, but as noted above it is far from bullet-proof. Other consequences include site names being longer than necessary in dropdown lists or selection panels. See IT Advanced Search or Current Site for examples.
To take this even further, we prepend the primary site name to the site name when adding clients to carriers in Claims Reporting in HGA. For example, 'primary site - subsite' example above would be listed as 'primary site - primary site-subsite'. This was likely done as a stop-gap because the first method was not bullet-proof and admins were still unable to find subsites reliably. Its a great idea, but should replace the first method instead of piling more site names on top of it.