Effects of "Full-Copy the files"

I implied but failed to specify that the "Backup Method for Files" selected on the GENERAL tab was "Full-Copy the files" for the occasional USB jobs. To test this I ran the offsite job, ran the policy Daily Diff job, disconnected the USB rive, and restored some files from the Differential. To my surprise the files restored fine, and timestamps on the B2D files and IMG folders show that the DD looked back past the Full Offsite job to the previous successful scheduled Weekly Full backup. The restored files existed in the WF and were not changed for the DD. Undocumented feature of the AVVI?