It's unusual that your minutes are being used up faster but they could be a reason for this.
Are you calling giffgaff numbers for over 60 minutes, any minutes above 60 will be taken from your inclusive minutes.
Also, if you get a goodybag from your airtime balance, your free giffgaff to giffgaff calls don't refresh!!
About the goodybag suggestion good one, perhaps giffgaff is making a note of feedback as they introduce new goodybags once in a while.