SEP Client version is different then Server

We have a strange issue regarding symantec version, the issue is the version of symantec on server different that of version deployed on Clients,and the symantec Liveupdate is regularly performing updates.

Server side version is SEP 11.0.6005.562          (Installed on Windows 2003 Server)

Client Side: SEP 11.0.7000.975                           (Installed on Windows 2008 R2 Servers)

is it install based on OS version?

Regards,

Agha