Why will the believer be held accountable again on the Day of Judgment, despite knowing his place in Paradise after being laid in the grave?

The aforementioned Hadith is authentic, and it states that when a servant is placed in his grave, he hears the sound of his companions' footsteps. Then, two angels come to him and ask him about the Prophet. The believer answers that he is Allah's servant and His Messenger. He is then told: "Look at your seat in Hellfire; Allah has exchanged it for you with a seat in Paradise." He sees both of them, and the believer's bliss and joy increase, while the disbeliever's sorrow, grief, and torment increase.

Accountability (Hisab) is one of the fundamental beliefs of Muslims. All people will be held accountable for their deeds, except for what has been exempted. The believer's reckoning will be easy; he will look at his book, and he will be forgiven. As for the disbeliever, his reckoning will be difficult.