Syntehtic Backup Incremental Cannot use DeDuplicated Storage

I'm just beggining to use dedupe in testing on BUE2014 and I've not really used it prior.

Basic testing of a Full+Incremental job to dedupe storage shows that both can use the same storage device without issue.

When I create a new synthetic backup, it allows me to select the dedupe storage device only for the full and synthetic full backups but not the incremental.

Why the difference? What is different about the synthetic that it's incremental cannot write to the same storage?

Thanks

Pete