And it is through that identification of himself with his Church below that, in the Eucharist, Our Lord is able to offer here in this world that Sacrifice of himself which he eternally offers in Heaven.

For at the Offertory of the Eucharist we offer ourselves and our daily lives to God, not as individuals separate from one another and separate from Our Lord, but as members together of his Body, intimately united to him.  In other words, at the Offertory we offer the Body of Christ in relation to ourselves.

But at the Consecration we offer to God Our Lord himself, present on our altar as he is in Heaven, and so take our part in that offering of himself which he eternally makes on our behalf.  In other words, at the Consecration we offer the Body of Christ in its relation to Christ himself.