Time Machine deleting backups from several months ago (but not the oldest)? During its "Cleanup" phase, Time Machine has recently been deleting backups from several months ago—even though these aren't the oldest backups, and my backup drive isn't full.

My backups go back to January 2012, yet just today, Time Machine deleted backups from May 21, June 11, July 2, and July 23 (all in 2012). And plus, my Time Machine drive still has 200 GB free.

Why's it doing this? Shouldn't it just either delete my backups from January 2012 first (if it needs the space), or just delete the expired backups in the past 24 hours or 30 days (based on its normal rules)?
According to Apple, Time Machine keeps backups 


hourly for the current 24 hours
daily for the last month
weekly copy until the disk is full


You may be noticing the system deleting "extraneous days", pruning the weeks' worth of daily backups down to the weekly copy.

As long as you can still view the weekly copies in Time Machine, they are still there.