The reason for the difference in dates has to do with misunderstanding the solar/lunar calculation of Ex. 12:2.
This month shall be unto you the beginning of months: it shall be the first month of the year to you.
How is that understood? Please note that verse refers to the lunar (months) and solar (year) measurements.
Year in Hebrew is shaneh. It means year as a division of time, of age, a revolution (return) of the sun. It has to first make a complete revolution to accurately mark the beginning of the months.
From Moses to Christ, it was known that the sun first had to have passed the spring equinox and then one looks for the new moon from which to count to the 14th day Passover (or as it too became muddled, the full moon of the 15th).
Here are quotes from Philo of Alexandria, a contemporary of Christ, on how they calculated the month and Passover.
THE THIRD FESTIVAL
XXVI. (140) Following the order which we have adopted, we proceed to speak of the third festival, that of the new moon. First of all, because it is the beginning of the month, and the beginning, whether of number or of time, is honourable.
The new moon is the beginning of the month. Next comes Passover on the fourteenth of that month.
THE FOURTH FESTIVAL XXVII. (145) And after the feast of the new moon comes the fourth festival, that of the passover, which the Hebrews call pascha, on which the whole people offer sacrifice, beginning at noonday and continuing till evening.
So, what of the vernal equinox? How does that enter the picture?
I will first jump to Josephus who says this about the year and month calculation of the beginning of the year.
- In the month of Xanthicus, which is by us called Nisan, and is the beginning of our year, on the fourteenth day of the lunar month, when the sun is in Aries, (for in this month it was that we were delivered from bondage under the Egyptians,) the law ordained that we should every year slay that sacrifice which I before told you we slew when we came out of Egypt, and which was called the Passover; and so we do celebrate this passover in companies, leaving nothing of what we sacrifice till the day following.
And now back to Philo who defines the same mark (vernal equinox) as he describes the Fifth Festival of Unleavened Bread.
This month, being the seventh both in number and order [the original order until changed at Ex. 12:2], according to the revolutions of the sun, is the first in power; (151) on which account it is also called the first in the sacred scriptures [Ex. 12:2]. And the reason, as I imagine, is as follows. The vernal equinox is an imitation and representation of that beginning in accordance with which this world was created. Accordingly, every year, God reminds men of the creation of the world, and with this view puts forward the spring, in which season all plants flourish and bloom; (152) for which reason this is very correctly set down in the law as the first month, since, in a manner, it may be said to be an impression of the first beginning of all, being stamped by it as by an archetypal Seal. ... And this feast [of Unleavened Bread] is begun on the fifteenth day of the month, in the middle of the month, on the day on which the moon is full of light, in consequence of the providence of God taking care that there shall be no darkness on that day.
And see here and here in which the latter states: "But this Aristobulus also adds, that for the feast of the Passover it was necessary not only that the sun should pass the equinoctial segment, but the moon also."
Thus we learn that to properly calculate the first month the sun had to be in Aries, the sun had to pass through the Spring Equinox, and then you look for the new moon. They didn't measure the beginning of the year from only the first full moon after the Spring Equinox. The correct calculation that made sure the sun was in Aries (after the Spring Equinox) necessitates when the intercalary month was added. The sun was in Aries, then they looked for the new moon, and thus the full moon.
As centuries past, the way to count became more muddled. By the time of Constantine (325 CE), the accusation became that the Jews were observing two Passovers in one (solar) year. This happened because they failed to count the solar year properly.
For the OP example shown in 2016, we see this problem in operation. For JW, they only counted the first full moon of 3/23/16 as the day of their memorial. This means the solar/lunar calendar would still be measuring the previous year 2015. In 2015, Passover fell on 4/4/15. For 2015, the sun had passed the Spring Equinox, the first new moon observed, and the full moon was 4/4/15. In 2016, the sun had not passed the Spring Equinox to mark the start of observing the first new moon and thus the full moon. Instead 3/23/16 (and Sunday 3/27/16) was merely the first full moon after the Spring Equinox. In this way, they mistakenly observed two Passovers in one year.
So, that's the reason for the "lag" so called. It's not that Easter was particularly miscalculated in 2016, but rather the JW (and Christian) observance was based on how later Jewish calculations came into play, rather than how Christ would have done it.
PS. To clarify, Easter was observed in 2016 on the first Sunday 3/27 following that first full moon on 3/23.
The Jewish Passover was observed 4/22.
If you love irony sharpening irony, this is a great example. In other words, the answer remains the same, but it's the switch. In Christ's time they observed the Passover on the first full moon after the first new moon after the Spring Equinox. That meant the sun had passed the Spring Equinox (in Josephus' parlance, it was in Aries). That's the correct way to calculate and thus observe the death, burial, resurrection of Christ Jesus.