More "explanations" hung on very small reference [my emphases]. I'm not at all sure.
Though the Trinity season, or the Sundays after Pentecost, was the last season of the ecclesiastical year for which lessons were appointed specifically for each Sunday, the Prayer Book tradition, nonetheless, has maintained and developed further the order and coherence of this season. This part of the church year, however, admits of a different character from that of the Advent to Pentecost sequence; a difference which the older commentators well understood ...
In the Roman Catholic Church, on the one hand, the season of Pentecost has undergone a number of changes, resulting in the dislocation of the order of the epistles and gospels. These changes, which were the result of gradual developments during the High and Late Middle Ages, became definitive and settled in the sixteenth century Counter-Reformation reforms [Guéranger: 116]. In general, the difference in the appointment of particular readings derives from the various ways in which the Sundays after Pentecost took shape, especially in relation to the accommodation of octaves and what the ancient ordines call Dominica vacat, the 'empty' Sundays after Ember Saturday Vigil ordinations with accompanying Mass [MacKenzie: 383 & 398]. The eucharistic lectionary of the Prayer Book, on the other hand, remained in critical continuity with the older western tradition through the Sarum Missal, thereby avoiding some of the later dislocations and preserving a more coherent set of propers.
The English church avoided the dislocation of epistles and gospels for the time between Pentecost and Advent that occurred in the Roman church [MacKenzie: 398]. Many of the problems claimed for in the season from Pentecost to Advent pertain to the order of readings found in their definitive form in the Roman lectionary from the sixteenth century onwards [Guéranger: 116]. They do not apply to the order of epistles and gospels found in the Sarum Missal and derived unto the Prayer Book. These dislocations explain the divergences between the pre-Vatican II Roman Church and the Church of England in the propers appointed for this part of the church year. The convergence of a number of factors perhaps provides something of an account for these differences.
What is now commonly known as Trinity Sunday, or the First Sunday after Pentecost, was anciently a Dominica vacans, being the Sunday immediately following the Pentecost Ember Saturday Vigil ordinations [MacKenzie: 398]. When this practice fell into disuse, there was need for the appointment of propers for the First Sunday after Pentecost. Thus Luke 6: 36-42, beginning with 'Be ye merciful, as your Father is merciful', which had been the gospel for the Fourth Sunday after Pentecost — popularly known as 'the Sunday of mercy' — became the gospel appointed for the First Sunday after Pentecost [Guéranger: 116]. Subsequently, the remaining gospels for the time after Pentecost were simply brought forward by one week; hence the dislocation of the epistles and gospels, especially for the first part of the season [Guéranger: 116 & MacKenzie: 398]. The Sarum Missal, however, avoided this dislocation. While this goes a long way towards explaining the divergences, it does not completely exhaust the complications.
The growing desire for the regular observance of the Feast of the Holy Trinity meant a gradual movement away from votive masses to the observance of the feast on the First Sunday after Pentecost [Guéranger, Vol. X: 91 ff. & MacKenzie: 398]. In England, the observance of this day as Trinity Sunday was established very early; in 1162 St. Thomas Becket of Canterbury instituted its celebration [Guéranger, Vol. X: 93]. Elsewhere in Europe the idea for the regular observance of the Feast of the Holy Trinity grew, but there was some variation as to the actual day appointed for its celebration.
Common observance was established in 1334 by Pope John XXII, who decreed the celebration of the Feast of the Holy Trinity on the First Sunday after Pentecost [Guéranger, Vol. X: 93]. The appointment of propers appropriate to that feast meant the displacement of those which had come to be read on the First Sunday after Pentecost. Consequently, the First Sunday after Pentecost was reduced to simply a commemoration at the Mass of the Feast of the Holy Trinity, the 'mercy' gospel, Luke 6: 36-42, appearing as the Last Gospel at High Mass instead of John 1: 1-14. But the epistles and gospels throughout the early part of the season remained in their dislocated order originally occasioned by the moving of Luke 6: 36-42 from the Fourth to the First Sunday after Pentecost.