Time Machine deleting backups from several months ago (but not the oldest)?
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.