I suppose the real reason for this is that there is no such thing as an objectively true account. Any first person narration, no matter how objective an author tries to make it, is bound to be unreliable to some extent. This is because that we are so grounded in our own world view and way of looking at life that we are not able to look upon what goes on around us without our own set of prejudices and values influencing our perspective on what happens.
Jim's narrative is therefore influenced by the way that he feels so strongly attached to the beautiful landscape around him, his own romantic sensibilities, his class attitude and the way in which he finds himself both attracted to and repulsed by the intriguing figure of Antonia in equal measure. All of these factors influence his narrative and limit his objectivity. The nature of a first person narrative is that we see what happens and the setting through the eyes of the character who describes it to us. This in turn influences our perception. The novel would have looked very different if it had been narrated to us by somebody who hated the sense of untamed wilderness expressed through the description of Nebraska.
Look at the character of Jim through the lens of your own experiences. How objective are you about the people you care about deeply: parents, friends, love interests, and heroes?
When it comes down to it, can you be “completely” objective about your experiences when you discuss your accounts with them, or do your experiences somehow influence your opinion?
It is very hard to be objective with the people we care the most about, and Antonia was very dear to Jim.