It sounds to me like it could be that the Cantor is singing the Psalm for this particular Mass. It could also be that it is a particular Psalm chosen for this Mass, as well.
I used to be a Cantor and a Psalmist in my last parish, and this is my best guess.
I tried to see if I could find anything on-line for you, but I didn’t find anything. Maybe someone else will come on and give a better answer.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.