The Chairman says:
In the Traditional Mass, the readings of each Mass are connected with the other proper prayers and chants of that Mass. There is a series of Sundays (some of the ones After Pentecost) in which, for historical reasons, the gospels have got out of sync with the other propers by one week; because of the nature of the progression of texts this doesn't matter very much. What we never have, however, in the Traditional Mass is a set of readings with absolutely no connection with the chants or prayers, and that is something which is inevitable with a multi-year cycle of readings, unless it were accompanied by the multi-year cycle of prayers and chants.
Is it then the case that ...
On the fourth Sunday after Trinity the Roman Lectionary omits Luke 6:36-42 from the list of readings and moves all subsequent Gospel readings up one Sunday, but without moving the corresponding Epistle readings.
Some Lutherans weren't sure.
I'm not sure either but the points of difference suggest that both reducing (as in the case of Sundays in Advent) and counting (after Epiphany, after Trinity) may have played a role.