Unchanged files repeatedly saved in differential backups

In successive differential backups, certain unchanged files are backed up in each run. The date, size, and CRC of the files are identical according to 7-zip.

For example, the same 400MB VirtualBox .vdi file keeps getting saved in every differential backup. Immediately after the original BaseFile.7z was created, Toucan was working correctly and was ignoring this file. As soon as the file changed, all subsequent differential backups have included a copy of this file even though it has stopped changing. (Virtualbox is not running).

How does Toucan determine whether a file has changed? Does it compare the file only against the original BaseFile or does it also check whether the file exists in the most recent differential backup and then compare it against that version as well?

EDIT: Apologies. It seems this behavior is correct. According to wikipedia, "A cumulative backup of all changes made since the last full or normal backup". I need an incremental backup - something like duplicity.