Can someone please help me with this, if you’ve looked into it before?
There are a several factors at work here:
1. JW's treat each year as independent of all the others, No leap years, no other holidays, events or festivals to integrate into a year to year calendar. (Once you set the date for one holiday, you've set the date for all the others throughout the rest of the year.) Sometimes there is nearly a month's discrepancy.
2. JW's, like most other fundamentalist groups believe Jesus died on the 14th, the exact same day the lamb was sacrificed. So on the afternoon of the 13th, JW's hurry home, get dressed and go to the Memorial. By the time they pass the emblems, the sun has set and the 14th has technically begun, sometimes by just a few minutes. The next day, when they wake up, it will still be the 14th.
In Judaism, the lamb is sacrificed on the afternoon of the 14th and Passover occurs after sundown on what is technically the 15th. (Numbers 33:3 clear says the next daylight period after Passover was the 15th, not the 14th.)
3. JW's like most fundy groups, believe they know more about the intricacies of the Law and the Jewish calendar (Both of which are quite complicated) than observant Jews do themselves. We've had some striking examples here on the forum in years past.
Hope this helps