Krishna learnt many arts and sciences which already has an answer here out of which some are related to conversation where he would have learnt different styles of speaking.
Bhagavata says in
tā dṛṣṭvāntikam āyātā bhagavān vraja-yoṣitaḥ avadad vadatāṁ śreṣṭho
vācaḥ peśair vimohayan
SB 10.29.17 — Seeing that the girls of Vraja had arrived, Lord Kṛṣṇa,
the best of speakers, greeted them with charming words that bewildered
their minds.
Hemadri says in his commentary to Mukta-phala of Vopadeva:
vedāḥ purāṇaṁ kāvyaṁ ca prabhur mitraṁ priyeva ca bodhayantīti hi
prāhus trivṛd bhāgavataṁ punaḥ
It is said that the Vedas instruct like a master, the Purāṇas instruct
like a friend, and poetic works instruct like a lover, but the
Śrīmad-Bhāgavatam does all the three.
From all the above, we can conclude that Krishna, being expert speaker, could chose different styles of speaking.
Two styles of speech are descriptive and prescriptive. Both have their usage, for example in post modern world, descriptive method is liked by people more than prescriptive and people may be more receptive to descriptive way rather than prescriptive.
I can think of two reasons:
First:
Krishna spoke some things in descriptive way and some in prescriptive way being expert speaker. But intention is same, to reveal the truth.
Second
Krishna doesn't have any issue if others explain the same truth in a different way.
We find an instance of this in Uddhava Gita:
SB 11.19.14 — I personally approve of that knowledge by which one sees
the combination of nine, eleven, five and three elements in all living
entities, and ultimately one element within those twenty-eight.
Uddhava later asks Krishna:
SB 11.22.1-3 — Uddhava inquired: My dear Lord, O master of the
universe, how many different elements of creation have been enumerated
by the great sages? I have heard You personally describe a total of
twenty-eight — God, the jīva soul, the mahat-tattva, false ego, the
five gross elements, the ten senses, the mind, the five subtle objects
of perception and the three modes of nature. But some authorities say
that there are twenty-six elements, while others cite twenty-five or
else seven, nine, six, four or eleven, and even others say that there
are seventeen, sixteen or thirteen. What did each of these sages have
in mind when he calculated the creative elements in such different
ways? O supreme eternal, kindly explain this to me.
Krishna replies:
SB 11.22.4 — Lord Kṛṣṇa replied: Because all material elements are
present everywhere, it is reasonable that different learned brāhmaṇas
have analyzed them in different ways. All such philosophers spoke
under the shelter of My mystic potency, and thus they could say
anything without contradicting the truth.
SB 11.22.5 — When philosophers argue, “I don’t choose to analyze this
particular case in the same way that you have,” it is simply My own
insurmountable energies that are motivating their analytic
disagreements.
SB 11.22.6 — By interaction of My energies different opinions arise.
But for those who have fixed their intelligence on Me and controlled
their senses, differences of perception disappear, and consequently
the very cause for argument is removed.
SB 11.22.7 — O best among men, because subtle and gross elements
mutually enter into one another, philosophers may calculate the number
of basic material elements in different ways, according to their
personal desire.
SB 11.22.8 — All subtle material elements are actually present within
their gross effects; similarly, all gross elements are present within
their subtle causes, since material creation takes place by
progressive manifestation of elements from subtle to gross. Thus we
can find all material elements within any single element.
SB 11.22.9 — Therefore, no matter which of these thinkers is speaking,
and regardless of whether in their calculations they include material
elements within their previous subtle causes or else within their
subsequent manifest products, I accept their conclusions as
authoritative, because a logical explanation can always be given for
each of the different theories.
How elements merge into each other or arise out of one another can be seen from the annihilation and creation process.
Process of annihilation of universe is described here:
SB 12.4.15-19 — The element fire then seizes the taste from the
element water, which, deprived of its unique quality, taste, merges
into fire. Air seizes the form inherent in fire, and then fire,
deprived of form, merges into air. The element ether seizes the
quality of air, namely touch, and that air enters into ether. Then, O
King, false ego in ignorance seizes sound, the quality of ether, after
which ether merges into false ego. False ego in the mode of passion
takes hold of the senses, and false ego in the mode of goodness
absorbs the demigods. Then the total mahat-tattva seizes false ego
along with its various functions, and that mahat is seized by the
three basic modes of nature — goodness, passion and ignorance. My dear
King Parīkṣit, these modes are further overtaken by the original
unmanifest form of nature, impelled by time. That unmanifest nature is
not subject to the six kinds of transformation caused by the influence
of time. Rather, it has no beginning and no end. It is the unmanifest,
eternal and infallible cause of creation.
