Thanks for the question.
The issue is really a matter of context. There are multiple Divine mysteries:
All of these realities are mysteries inasmuch as we can never fully comprehend their depth, because only God can sound the depths of God.
That being said, through revelation we have gained access to these mysteries, in Christ Jesus. The Mass is highlighting that which pertains to His act of Redemption, and our participation in that mystery. Keep in mind that in the Novus Ordo there are three formulas:
(What you cited above is an older form, the current form is below)
- We proclaim your death O Lord and profess your Resurrection until you come again.
- When we eat this bread and drink this cup we proclaim your death O Lord, until you come again.
- Save us Savior of the world for by your Cross and Resurrection, you have set us free.
All of these attempt to express an aspect of the profound mystery to which we belong in Christ. So, in the end the difference is a matter of context and what is being highlighted.