For more information please see https://support.code42.com/CrashPlan/6/Code42_app_reference/Code42_app_reference
Why the amount of space stored on the server and on your local computers are different
CrashPlan uses a process called deduplication, a data compression technique used to conserve storage space. CrashPlan first shrinks your data by compression and then only backs up block level changes to the original data. In addition a 90 day history of the changes is preserved. This is why your total server storage are often larger than your total files selected.
To check your disk space usage please see this page: