Think what they are saying is you would only ever have one differential backup
- Differential = since last full backup
- Incremental = since last incremental
Therefore if a file changes twice then it could be backed up twice by incremental, but only once by differential. Differential will only save essentially the latest version of file.
EDIT: For wikipedia reference
Microsoft, makers of Backup and Restore, defines incremental backups and differential backups as follows: "Incremental: An incremental backup provides a backup of files that have changed or are new since the last incremental backup....Differential: A differential backup provides a backup of files that have changed since a full backup was performed. A differential backup typically saves only the files that are different or new since the last full backup, but this can vary in different backup programs. Together, a full backup and a differential backup include all the files on your computer, changed and unchanged."