@JulienBlanc, as @nephros highlights, it looks like the bug part of your bug report (that the backups are performed at the sync time, rather than the back up time) is covered by the earlier report from @Cryx .
The suggestion to purge older backups is certainly a sensible idea, but it really belongs in the Feature Request section. For the sake of keeping things clean and clear, I’ll close this thread with the hope that further discussion about the bug can take place on the earlier thread. I’d be grateful if you could create a new post about purging backups in the Feature Request section.
If I’ve misunderstood, or you would like to keep this topic open anyway, then please send me a direct message and I can open it up again.