In Genesis 8:21 God thinks to himself:
וַיָּ֣רַח יְהוָה֮ אֶת־רֵ֣יחַ הַנִּיחֹחַ֒ וַיֹּ֨אמֶר יְהוָ֜ה אֶל־לִבּ֗וֹ לֹֽא־אֹ֠סִף לְקַלֵּ֨ל ע֤וֹד אֶת־הָֽאֲדָמָה֙ בַּעֲב֣וּר הָֽאָדָ֔ם כִּ֠י יֵ֣צֶר לֵ֧ב הָאָדָ֛ם רַ֖ע מִנְּעֻרָ֑יו וְלֹֽא־אֹסִ֥ף ע֛וֹד לְהַכּ֥וֹת אֶת־כָּל־חַ֖י כַּֽאֲשֶׁ֥ר עָשִֽׂיתִי׃
The LORD smelled the pleasing odor, and the LORD said to Himself: “Never again will I doom the earth because of man, since the devisings of man’s mind are evil from his youth; nor will I ever again destroy every living being, as I have done.
Then a few lines later in Genesis 9:11 (same link as above), God says to Noah:
וַהֲקִמֹתִ֤י אֶת־בְּרִיתִי֙ אִתְּכֶ֔ם וְלֹֽא־יִכָּרֵ֧ת כָּל־בָּשָׂ֛ר ע֖וֹד מִמֵּ֣י הַמַּבּ֑וּל וְלֹֽא־יִהְיֶ֥ה ע֛וֹד מַבּ֖וּל לְשַׁחֵ֥ת הָאָֽרֶץ׃
I will maintain My covenant with you: never again shall all flesh be cut off by the waters of a flood, and never again shall there be a flood to destroy the earth.”
Paraphrasing, God thinks, I will never destroy all humankind; God says, I will never destroy all humankind with a flood.
The Lubavitcher Rebbe (an exceptional Torah scholar) maintains (see "concise" in the index of "Torah Studies a parsha anthology" for multiple references) that extra or superfluous words are avoided in the Torah. Hence, adding the words "with a flood" to what God says to Noah must be significant. What God says to Noah is much more qualified or limited than what He thought earlier.
Why the difference? What does this difference teach us?