Why would some backup schedules work and others not?

Question

I have several backup schedules set up:

• Every 15 minutes (keeping 12)
• Every 8 hours (keeping 9)
• Daily Backup (keeping 31)
• Weekly Backup (keeping 52)
• Yearly Backup (keeping 5)

The perplexing problem is that some of them seem to be working and some not, even though they're all using the default profile, and they're all enabled, and they're all being saved to the scheduled backups directory.

Here are the backups that are appearing:

sitename-2012-11-10T18-10-05 Sat, 11/10/2012 - 18:10 25 min  311.12 KB 
sitename-2012-11-10T18-03-58 Sat, 11/10/2012 - 18:03 31 min  305.58 KB 
sitename-2012-11-10T17-48-49 Sat, 11/10/2012 - 17:48 46 min  308.19 KB 
sitename-2012-11-10T16-43-06 Sat, 11/10/2012 - 16:43 1 hour  303.99 KB 
sitename-2012-11-10T16-43-05 Sat, 11/10/2012 - 16:43 1 hour  303.99 KB 
sitename-2012-11-10T15-43-02 Sat, 11/10/2012 - 15:43 2 hours 304.01 KB  
sitename-2012-11-10T14-43-00 Sat, 11/10/2012 - 14:43 3 hours 303.99 KB 
sitename-2012-11-10T13-38-00 Sat, 11/10/2012 - 13:38 4 hours 303.95 KB 
sitename-2012-11-10T12-33-00 Sat, 11/10/2012 - 12:33 6 hours 303.95 KB 
sitename-2012-11-10T11-28-01 Sat, 11/10/2012 - 11:28 7 hours 303.94 KB 
sitename-2012-11-10T10-23-00 Sat, 11/10/2012 - 10:23 8 hours 303.95 KB 
sitename-2012-11-10T09-18-02 Sat, 11/10/2012 - 09:18 9 hours 303.94 KB 

The fact that there are 12 (which is the number mentioned in one of the schedules) makes me think that perhaps the Backup & Restore module has a major bug: perhaps it assumes you'll only have one or two schedules, and my 5 overwhelmed it.

Is there any other reason for the behavior you're seeing?