This is what seems to have happened in TeflonWoman's case. She joined in November, but didn't take her first flight until March. TP collection year end then changed from 8 Dec to 8 Apr.

She has queried this with BAEC (as T&Cs seem to suggest join date determines year end date), but has not received a reply after now four weeks. We wanted our respective collection years to co-incide (hence the "early" join date), but may have been too clever by half ...