'Size of items ready to archive' Vs 'Size of messages that would be archived'

Hi all,

I have run an Archiving report against a single mailbox and I would like to understand the difference between the columns: 

I have read the following tech notes and am still unclear on why this is happening.

http://www.symantec.com/business/support/index?page=content&id=TECH35622

http://www.symantec.com/business/support/index?page=content&id=HOWTO37063

I could understand it if the first column was higher that the second because the archiving might not be able to do a full pass on the mailbox.

Many thanks,

Graham