It funny, as Dec are most companies finance closing month. Hence the logic shall be align.
Keep in mind that when someone's AMF is due will be different, as its based on when you get your first ISC2 cert. Not everyone will need to pay in December.
This is different from ISACA, where they want everyone to renew at the end of the calendar year.
Michael Brown, CISSP, HCISPP, CISA, CISM, CGEIT, CRISC, CDPSE, GSLC, GSTRT, ISSA Fellow