Luke 3:1-3 was Luke mistaken about the year in which John the Baptist’s preaching began?

Luke goes into detail concerning the beginning of Christ’s ministry. To quote:

“In the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate
being governor of Judea, and Herod being tetrarch of Galilee, and his
brother Philip tetrarch of the region of Ituraea and Trachonitis, and
Lysanias tetrarch of Abilene, during the high priesthood of Annas and
Caiaphas, the word of God came to John the son of Zechariah in the
wilderness. And he went into all the region around the Jordan,
proclaiming a baptism of repentance for the forgiveness of sins.”
(Luke 3:1-3)

Tiberius succeeded his father, Augustus Caesar, who died 19th August, AD 14, so if that is our benchmark, John’s preaching would have begun in AD 28, with Jesus’ ministry beginning early AD 29.
However, one of the early Church Fathers, Tertullian, had this to say:

“… the Lord has been revealed since the twelfth year of Tiberius
Caesar.” (Against Marcion, Book 1, ch. 15)

So, if Tertulian’s date is our benchmark, John’s preaching would have begun in AD 26, with Jesus’ ministry beginning early AD 27. Now, Tertullian wrote this after Luke’s gospel, so he must have known what Luke had said. Why the difference? Was one of them mistaken, or is there another explanation?