I noticed that inside Bing webmaster tools, they are considered all information relating to all third-level subdomains.

Example:
dominioprincipale.com
sub1.dominioprincipale.com
sub2.dominioprincipale.com
sub3.dominioprincipale.com
sub4.dominioprincipale.com

Why does this happen? Why Bing considers a single website?