Gday mate,
That could be because WP-Cron was processing a different task when you scheduled the instant backup.
It does not like doing to many things at once so sometimes it can take a few minutes for an "instant backup" to start. Hmm... maybe I can add some sort of feedback to let users know there will be a delay.
How did you go. Did you get a successful backup completed?
Cheers,
Mikey