Activity Monitor shows the total size of the data protected. 
OpsCenter under Monitor - Jobs shows the actual amount of data written to the storage unit. 
PureDisk and MSDP storage units do not save data that they already have backed up.  Therefore, it is likely that the amount of data actually written to the storage unit is less than the total amount of data being protected.  Because of this OpsCenter will show less data than Activity Monitor does. 

To see this create a custom tabular report in OpsCenter.   Include the following fields along with the other fields you desire:  Backup Job Primary ID, Backup Job Protected Size, and Backup Job Size.
The output of this new report will show that the Backup Job Protected Size field matches Activity Monitor and the Backup Job Size field matches Monitor - Jobs.