delta is the difference in the signature (virus/ips) definition size of the one that is present on the client to the one that is present on SEPM.
Delta is formed when the base defiition ( running on client) is old and the SEPM has the new set as well as the definition as that on client. The SEPM starts creating delta based on these definitions.
this information should be of help
http://www.symantec.com/business/support/index?page=content&id=TECH131528&locale=en_US