The formula JWs adhere to for determining Nisan 14 is that the new moon closest to the spring equinox as seen from Jerusalem is determined to be Nisan 1. 14 days later is Nisan 14. They are consistent in holding to his formula regardless of the Jewish calendar, or when Good Friday, or Easter are observed.
I can only guess that they put themselves in the position of someone who attended the Last Supper in 33 CE and do the math from there. I think they are less concerned about accuracy than they are stubborn about consistency with their own tradition.