That is one of the key differences in backup solutions that go off-site, can they recognize and do diff's (differential) backups as opposed to re-copying entire files.
This is really useful for keeping backups of whole directories, while only updating on the off-site backup the files that have changed. (say accounting, generated correspondence, training files, templates, images)
In the case of AC backups, unless you are running a script to replace-rename the most recent backup, you will get full file writes each night.
