Maybe not so much a coincidence then? You'd think both of these orbital alignments are a little beyond our control. Or maybe there was enough fuel budget both times to get +/- a couple days to get the insertion dates we wanted.

Yes, there's usually enough flexibility to move things around a bit. Robert Farquhar, the man who planned the trajectory of ISEE-3 (the craft re-contacted by amateurs during an Earth flyby two years ago) enjoyed arranging his missions so that important milestones happened on the birthdays or anniversaries of friends and family.