The Feast of the Baptism of the Lord (and the other feasts of the Lord) have two sets of readings for each year.
In the first edition of the Lectionary, issued in 1969, each of the feasts of the Lord have the same first (Old Testament) and second (New Testament) reading every year, but the Gospel is taken from Matthew, Mark, or Luke, depending on whether the year is A, B, or C.
In the second edition, issued in 1981, a complete set of readings is provided for each year, A, B, and C. But the selection of readings from the first edition is still an option.