In perfect Catholic form, the Catechism of the Catholic Church speaks pretty directly, but somewhat ambiguously if not paradoxically, on interpreting scripture and the Gospels in particular.
Firstly, scripture is still "open" for various and new understanding.
Still, the Christian faith is not a “religion of the book.”
Christianity is the religion of the “Word” of God, a word which is
“not a written and mute word, but the Word which is incarnate and
living.” If the Scriptures are not to remain a dead letter,
Christ, the eternal Word of the living God, must, through the Holy
Spirit, “open [our] minds to understand the Scriptures.” (CCC
This doesn't grant us freedom to read into scripture whatever we wish. Our interpretation must fit within the existing framework and beliefs of the Church. But, it does support the idea that various truths can be extracted from the parables within reasonable bounds.
With specific regard to the Gospels, we're fed two slightly contradictory messages:
We can distinguish three stages in the formation of the Gospels:
The life and teaching of Jesus. The Church holds firmly that the four Gospels, “whose historicity she unhesitatingly affirms,
faithfully hand on what Jesus, the Son of God, while he lived among
men, really did and taught for their eternal salvation, until the day
when he was taken up.”
The oral tradition. “For, after the ascension of the Lord, the apostles handed on to their hearers what he had said and done, but
with that fuller understanding which they, instructed by the glorious
events of Christ and enlightened by the Spirit of truth, now enjoyed.”
The written Gospels. “The sacred authors, in writing the four Gospels, selected certain of the many elements which had been handed
on, either orally or already in written form; others they synthesized
or explained with an eye to the situation of the churches, while
sustaining the form of preaching, but always in such a fashion that
they have told us the honest truth about Jesus.”
Furthermore (actually stated earlier):
In Sacred Scripture, God speaks to man in a human way. To interpret
Scripture correctly, the reader must be attentive to what the human
authors truly wanted to affirm and to what God wanted to reveal to us
by their words. (CCC 109)
On the one hand, we're told that the Gospels are true to the actual events. On the other hand, we're told that details were included and possibly fabricated to aid in our understanding. That in mind, I think the most reasonable understanding is that the events actually occurred, that Christ clarified things for his disciples, but that all truths revealed by the parables were not necessarily written in the Gospels.
What the CCC has to say with specific regard to the parables supports this notion:
Jesus’ invitation to enter his kingdom comes in the form of parables,
a characteristic feature of his teaching. Through his parables he
invites people to the feast of the kingdom, but he also asks for a
radical choice: to gain the kingdom, one must give everything. Words
are not enough; deeds are required. The parables are like mirrors for
man: will he be hard soil or good earth for the word? What use has he
made of the talents he has received? Jesus and the presence of the
kingdom in this world are secretly at the heart of the parables. One
must enter the kingdom, that is, become a disciple of Christ, in order
to “know the secrets of the kingdom of heaven.” For those who stay
“outside,” everything remains enigmatic. (CCC
Those who wish to enter the kingdom, the disciples of Christ, will given the secrets of heaven. It's not at all a stretch to assume, then, that those closest to Christ were given specific explanation -- as the Gospel says.
TLDR: My best understanding is that Christ clarified the parables in the greatest detail necessary to the faithfully seeking disciples; but that each Gospel author recorded only those explicitly revealed interpretations He was aware of (obviously) and which were relevant to the community he wrote to.
And for what it's worth, I do not suspect the the Gospel writers "synthesized" Christ's actual words; more likely for synthesis are the number of loaves and fish fed to an edited number of people on a hill.