Why is the Responsorial Psalm sometimes not a psalm?

[CatholicZ09]

This has been something that’s periodically been on my mind, so I figured I would ask here once it popped into my head again.

Why is the Responsorial Psalm in Mass sometimes not a psalm? For example, on the Third Sunday of Advent, Year B, our “psalm” is the Magnificat from the Gospel of Luke, “My soul rejoices in my God.” In Year C, the psalm for the very same Sunday is from Isaiah 12, “Cry out with joy and gladness…” In Year A, the “psalm” for the Solemnity of the Most Holy Trinity is from Daniel 3, “Glory and praise forever.”

Is there any reason why some days in the Lectionary stray from a typical “psalm” and use something else, instead? Is the “Responsorial Psalm” named as such because a psalm is usually what is proclaimed at this point during Mass, but that doesn’t necessarily mean what is proclaimed is always a psalm?

Pardon my ignorance, but it’s just something that’s always perturbed me.

 

[Vico]

GIRM (US) See the last paragraph.

The Responsorial Psalm
61. After the First Reading follows the Responsorial Psalm, which is an
integral part of the Liturgy of the Word and which has great liturgical and
pastoral importance, since it fosters meditation on the Word of God.

The Responsorial Psalm should correspond to each reading and
should usually be taken from the Lectionary.

It is preferable for the Responsorial Psalm to be sung, at least as far
as the people’s response is concerned. Hence the psalmist, or cantor of the
Psalm, sings the Psalm verses at the ambo or another suitable place, while
the whole congregation sits and listens, normally taking part by means of
the response, except when the Psalm is sung straight through, that is,
without a response. However, in order that the people may be able to sing
the Psalm response more easily, texts of some responses and Psalms have
been chosen for the different times of the year or for the different categories
of Saints. These may be used instead of the text corresponding to the
reading whenever the Psalm is sung. If the Psalm cannot be sung, then it
should be recited in a way that is particularly suited to fostering meditation
on the Word of God.

In the Dioceses of the United States of America, instead of the Psalm
assigned in the Lectionary, there may be sung either the Responsorial
Gradual from the Graduale Romanum, or the Responsorial Psalm or the
Alleluia Psalm from the Graduale Simplex, as described in these books, or an
antiphon and Psalm from another collection of Psalms and antiphons,
including Psalms arranged in metrical form, providing that they have been
approved by the Conference of Bishops or the Diocesan Bishop. Songs or
hymns may not be used in place of the Responsorial Psalm.

 

[runningdude]

“Psalm” means a liturgical song. The Book of Psalms contains ancient Jewish liturgical songs, but they are not the only possible songs appropriate for the liturgy. Psalms can be taken or adapted from any part of scripture.

 

[TomH1]

Because other portions of the Holy Scriptures can be used in place of an actual psalm. This happens a lot more frequently in the Divine Office. A portion of scripture sung (preferably) or recited other than from the Book of Psalms is called a canticle.