Mora,

Welcome to the Forum.

I don't have the Latin text in front of me, but I would guess (from the English I do have, and my best effort at reverse engineering) that one of three things is true:

1) The English translators use three expressions "liturgical services", " liturgical celebrations" and "liturgical action" in an attempt to avoid repeating the same expression, with or without cause.

2) In the mind of the translators, "actionibus liturgicis" should be rendered with "liturgical services" as a way of avoiding saying "Mass", which is what most people think of (erroneously) as the whole of "THE liturgy", since the intent is not to limit Gregorian chant to Mass alone. There might (also) be an ecumenical degree here, trying to avoid "Mass" -- which, again, is clearly not the only liturgical action -- so it sounds less, um, exclusivist -- or something like that.

3) The people who wrote the documents themselves are known to have inserted phrases so that they could be used later to do more work than they appeared. Bugnini admits this, and Michael Davies claims it with good cause, but it may not be the explanation in this particular instance.

Action is a very important word. The liturgy is, first and foremost the action of Christ. What we do are actions that support that of Christ. This is far more than "service."