It makes sense because you lose 2nd Vespers if you go to the Saturday, and only 1st Vespers if you go to MondayUniversal Norms on the Liturgical Year and the General Roman Calendar
fourteenth day of the month of February in the year 1969,
5. Because of its special importance, the celebration of Sunday gives way only to Solemnities and Feasts of the Lord; indeed, the Sundays of Advent, Lent and Easter have precedence over all Feasts of the Lord and over all Solemnities. In fact, Solemnities occurring on these Sundays are transferred to the following Monday unless they occur on Palm Sunday or on Sunday of the Lord’s Resurrection.
We establish that they will go into effect on January 1, 1970, according to the decrees which will be published jointly by the Sacred Congregation of Rites and Consilium, and which will be valid until the edition of the restored Missal and Breviary.
Annunciation and St Joseph can be transferred to a day after Low Sunday, but not to a day that has a feast (d2) so it could fall on the Tuesday etc. So 1st Vespers of St. Joseph could be needed.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.