God, through the pages of the Bible, is progresively unveiling the truth about Himself.
He hates sin. But that is not all - that He only hates sin.
He also wants to unveil His omniscience in every man's condition. Not just the hatred for sin we need to see. We also need to see other aspects of God.
Jonah is a book unveiling the reluctance of God to have to judge a sinful people.
The last sentence of
Jonah demonstrates that God knew to the NUMBER, how many people should be exempted in the condemnation of that society.
This was progressive revelation. Not a change in God's mind about Nineveh's sins, but indication of His awareness of the vested interest of less than pure prophets. And also it shows God's infallible knowledge of the exact circumstances of all the people in the city in danger of judgment.
Jonah's last verse:
" And Jehovah said, You had pity on the tree that you did not labor for nor cause to grow, which came into being overnight and perished overnight;
And I, should I not have pity on Nineveh, the great city, in which are more than a hundred and twenty thousand people who cannot discern between their right hand and their left, and many cattle?" (Jonah 4:11)
In judgment God is infinitely knowledgeable and precise. And He can have great pity.
But He is also absolutely righteous.
Not a change of mind but a further revelation of His nature.