Warning, Church Geek Rant ahead.
At one time, the Sundays after Easter were counted (in the west) as X number of Sunday after certain feasts. "After Pentecost", "After Apostles" (the feast of Peter and Paul on 29 June), "After Laurence" (Feast of St Laurence on 10 August) and "After the Angels" (St Michael and All Angels on 29 September). Advent being the next season was, in some places, the Sundays "After St Martin" on 11 November. In some places it was the 4 Sundays before Nativity. This was a lot of counting! It was regularized in Rome, at least, to Sundays after Pentecost sometime before the 15th Century. In much of Europe it was regularized to "Sundays after Trinity" - being the first Sunday after Pentecost. In the time before the Reformation and the Council of Trent, many local churches also had their own calendar and their own cycle of Bible readings, called a lectionary. One of the more famous of the period (but only so famous in our day) is the Sarum Lectionary, being the the use of the Salisbury Cathedral in England. They counted Sundays after Trinity. This is important because it provides one of the sources for modern Western Rite liturgical calendars and readings.
At the Council of Trent the Roman church did things to her liturgy. First: she suppressed almost all the local variations and ordered all Roman Catholic churches to use the same liturgy. Second: she ordered all churches to follow her tradition of counting Sundays after Pentecost. This might sound like a matter of merely counting different (Pentecost is one Sunday before Trinity, so just add one) except for the next item. Third: she moved the collects in one direction in time and Gospels were moved in the other direction.
This means that what the prayers and readings said, formerly on (eg) the fourth Sunday after Trinity is now broken out and said on the third, the fourth, and the fifth Sundays.
Where this comes home, as it were, is in our Western Rites (ROCOR and Antioch, New Calendar and Old): because the After Pentecost counting happened after the Reformation, the Anglican tradition - and thus the Rite of St Tikhon - following the Use of Sarum Cathedral, still uses Sundays after Trinity. The difference between the Tikhon liturgy and those who use the Liturgy of St Gregory (based on the Roman Rite) is not just a difference of one Sunday. If one is inclined (as I am) to build connections and meditations on the liturgy based on its component parts, you get a different mosaic from Tikhon than you do from Gregory.
This Church Geekery is compounded by the fact that almost all WR liturgical materials are produced for both sets of communities. You cannot simply subtract one (or add one) to transpose between liturgies.
End of Rant.