Textual variants.

TEXTUAL VARIANT 'tɛks.tʃ(əw.)əl 'vɛr.i.ənt noun Form or version of a document which differs in some respect from other copies or editions of the same document.

Before the printing press was invented in the 1400s, books were copied by hand.

Sometimes this was done carefully and conscientiously. The Masoretes, fr’instance, were Jewish scholars who wanted to be certain they got exact copies of the scriptures, with super-duper anal-retentive precision. So they invented a very careful procedure, including a system of checksums, to be sure every copy of the bible was an exact replica. It’s why, when you compare the first-century Dead Sea Scrolls with 10th-century copies of the Old Testament, you find astonishingly few differences. Dudes knew what they were about.

Other times, not so much.

Even when they knew this was a very important book. (Heck, back then most books were considered important. Hand-copying meant publishing was crazy expensive.) Copyists had a bad habit of duplicating books in a rush. Popular books were occasionally copied in a group: You get a roomful of scribes, one of whom slowly dictated the “original,” and the rest of whom wrote it down en masse. Naturally mistakes would happen.

Which was no surprise to any literate ancient: People make mistakes. An ancient Christian would assume if this was a verse they’d never heard before, or one they’d learned differently, it must be some scribe’s mistake. Fr’instance the Egyptian commentator Origen (185–254), in his commentary on John (my translation):

203 “These things happened in Bethabara beyond the Jordan, where John was baptizing.” Jn 1.28 204 Yes, it’s indeed printed in all the copies, “These things happened in Bethany.” We’re not ignorant it’s like this, and got this way long ago: We’re well aware it’s “Bethany,” according to Irakléon. But we’ve come to the conclusion it shouldn’t be “Bethany” but “Bethabara”—we’ve been to these places, following the history of the footsteps of Jesus, his students, and the prophets. 205 This evangelist declares Bethany is the hometown of Lazarus, Martha, and Mary, about 15 stadia [2.8 km] from Jerusalem. There isn’t any same-named Bethany in the area of the Jordan. They pointed out Bethabara, by the Jordan’s banks; our inquiries found that John baptized there. Origen, John 6.24

Yep, Origen went to Judea, and his tour guides told him there wasn’t any Bethany near the Jordan, then pointed him to Bethabara, convinced him this was the right place, and probably sold him a few souvenirs. I once had some folks in Israel try to similarly convince me about the location of Jesus’s sepulcher, among other “biblical” sites they built churches atop.

So was Origen right? Nah. Thanks to archeology, we know there was another same-named Bethany on the east bank of the Jordan. (Today it’s called al-Maghtas, Jordan.) Hence our current editions of the Greek NT stuck with the Βηθανία/Vithanía, “Bethany,” which Origen groused was in all his copies of John. Most of our current translations follow suit.

The few who don’t are going off the Textus Receptus, which has Βηθαβαρᾷ/Vithavará (KJV “Bethabara”). That’s because Origen managed to convince some folks he was correct—and the editor of the Textus, Desiderius Erasmus, was one of ’em. Since the King James Version used the Textus as its baseline, that’s what we find in the KJV and NKJV. Jn 1.28 NKJV

So there y’go: Two ways variants happen. Copyists, in their haste, slip up; and know-it-all interpreters rejigger the original to suit themselves.