Why did the Prophet (peace be upon him) seek forgiveness for error, even though error has been lifted from his Ummah?

The aforementioned Hadith is authentic, narrated by Muslim, and its wording is: "O Allah, forgive my sin and my ignorance, and my extravagance in my affairs, and what You know better than I. O Allah, forgive my seriousness and my jest, my mistakes and my intentions, and all that is with me. O Allah, forgive me for what I have put forward and what I have delayed, and what I have concealed and what I have made manifest, and what You know better than I. You are the Expediter and You are the Delayer, and You are over all things competent."

The Prophet's (peace be upon him) seeking forgiveness, despite his past and future sins being forgiven, is an act of worship, an increase in his ranks, and a teaching for his nation to emulate him, so that seeking forgiveness encompasses everything a servant has done, knowingly or unknowingly.

Some scholars have interpreted his seeking forgiveness as being due to the "ghan" (a type of spiritual veil or rust) that afflicts the heart from preoccupation with creation, so he resorts to seeking forgiveness to request the previous state of complete proximity to Allah.