How does Allah rejoice at His servant's repentance when He already knows that he will repent, as opposed to the joy of humans over something they did not expect?

The exact wording mentioned for the Hadith (definition) was not reported, but it was reported in the Two Sahihs (Bukhari and Muslim) that the Prophet, peace and blessings be upon him, said: "Allah is more pleased with the repentance of His servant when he repents to Him than one of you who was on his riding animal in a barren land, and it escaped from him, carrying his food and drink. He despaired of finding it, so he came to a tree and lay down in its shade, having despaired of his riding animal. While he was in that state, behold, he found it standing beside him. He took hold of its rein and then said, out of extreme joy, 'O Allah, You are my slave, and I am Your Lord!' He made this mistake due to extreme joy."

Allah’s joy is a divine attribute befitting Him, glorified be He. He is all-knowing, and His joy does not entail any imperfection that might befall the joy of creation. The Ahl al-Sunnah (definition) wal-Jama'ah (Sunni Muslims) believe in Allah's attributes without likening them to creation or trying to explain their 'how,' as Allah the Almighty says: "There is nothing like unto Him, and He is the All-Hearing, the All-Seeing."