As others have stated, this is a modification that was made by John XXIII in the 1962 missal.
The other three Ember Days are relatively easy to calculate, given that they follow a specific Sunday (Third Sunday in Advent, First Sunday in Lent, and Pentecost). The autumn Ember Days, however, are a bit more complicated, because they follow the "third liturgical Sunday of September," which, believe it or not, is not as simple to figure out as it might seem.
Under the "old" was of calculating things, the first liturgical Sunday in September didn't actually have to occur in September; rather, it was the Sunday that occurred during the first week of September. So, for example, this year, the first liturgical Sunday of September was actually August 30. This would make the third liturgical Sunday of the month September 13, and thus the Ember Days would be the 16th, 17th, and 19th.
John XXIII modified this so that liturgical Sundays and calendar Sundays became the same thing. So, the Ember Days followed the actual third Sunday in September, which was September 20, making the Ember Days the 23rd, 24th, and 26th.
A lot of people believe that the autumnal Ember Days are connected with the Feast of Exaltation of the Holy Cross, but this is not officially correct. It is an often-cited rule of thumb, but officially the timing of the Ember Days has always been connected with the third Sunday of September. It just a question of how you calculate what the third Sunday actually is.