Variable amount payment denied on resignation

In my previous company financial year is Jan-Dec Calender year, also the CTC is paid accordingly with Variable amount based on the whole year performance paid in the month of Feb for previous year. its also shown in the CTC calculation. I resigned in the month of October and left organisation in Jan-2018, so I was ideally on pay-roll for complete year and now HR is saying I'm not entitled for the variable amount payment as per their HR policy. Could you please help, is it legal to have such HR policies?