There are unusual references in the Zohar to Sages who came much later than Rabbi Shimon bar Yochai. There are mentions of Rabbi Yochanan and Resh Lakish, who lived a few hundred years later, and even to the Sages called Geonim (geniuses) who lived almost a thousand year after Rabbi Shimon.
Here are some explanations that I've read at 5:30 AM, a very right time for such things, waiting to board the plane back to Houston.
After Rabbi Shimon bar Yochai and his students died and entered Gan Eden, they kept a running commentary on the Zohar, adding to it as they saw fit. The book was in the possession of a few select Sages, but they kept discovering new things in it. This is no doubt real magic, but why this should not be the case, since they were all wizards during their lifetimes? (Mikdash Melech)
Rabbi Shimon kept revealing secrets to his students after his death. That is the “later composition” as opposed to “first composition” mentioned in the Zohar. The parts about the later Sages were added when these Sages were already living (Sichot HaRan, Rabbi Nachman of Breslov).
Rabbi Abba, who was tasked by Rabbi Shimon with writing down the teaching, lived for 300 years. He transmitted the book to Rav, when Rav traveled to Babylon, where he revealed it to select Sages, and later it came into the hands of the Geonim. They kept adding commentaries into the text, because the nature of the teaching is that it is a living book (some find problems with this explanation, because of the lack of attributions).
The complete Zohar was authored by Rabbi Shimon bar Yochai, no excuses, but he knew the future and wrote the teaching of the Sages who were to come later. For Rabbi Shimon, there was no difference between the past and the future, and he quoted from both (Rabbi Chaim Vital). (My commentary: it was important for the book to be hidden all this time, because it should not exert undue influence on future events that are described in it.)
These explanation are from the book “Shaarei Zohar” by Rabbi Daniel Frisch, who in his translation of the Zohar called “Matok MiDvash” (Sweeter Than Honey) prefers the practical explanation, and marks stories from later time (such as those of Rabba bar bar Channah) as external additons.
Art: Louis Charles Moeller - Stubborn