This is how God can be seen as being "especially" present in the heart of a believer, or "where two or three are gathered," or on Sinai's height, or on Mt. Tabor, or in the gathered congregation, or in the womb of His Blessed Mother, or in the Holy Eucharist, or in Heaven with His Saints.
See? God is present in those places (as in all places), but not in exactly the same way.
It is not He who varies. It is we who vary.
We are not equally present to HIM,for instance, if we are drunk or crazy or unconscious or resistant to Him. But HE is present always.
It's like Augustine said (and I really love this),
Late have I loved you, Beauty so ancient and so new,
late have I loved you!
Lo, you were within,
but I outside, seeking there for you,
and upon the lovely things you have made
I rushed headlong – I, unlovely.
You were with me, but I was not with you.
They held me back far from you,
those things which would have no being,
were they not in you.
You called, shouted, broke through my deafness;
you flared, blazed, banished my blindness;
you lavished your fragrance, I gasped;
and now I pant for you;
I tasted you, and now I hunger and thirst;
you touched me, and I burned for your peace.
To me, that's a WOW.
He’s already with the believer......at least in Christianity. Not sure about Roman Catholicism.