Why did Subhūti not have the thought "I am an arhat free from desire"?

Original text of the Diamond Sutra: 世尊。佛说我得无诤三昧。人中最为第一。是第一离欲阿罗汉。我不作是念。我是离欲阿罗汉。世尊。我若作是念。我得阿罗汉道。世尊则不说须菩提是乐阿兰那行者。以须菩提实无所行。而名须菩提是乐阿兰那行者。

A literal translation is as follows: Subhūti, in order to corroborate that an arhat has no thought of "I am an arhat," said to the Buddha: World-Honored One, the Buddha has said that I have attained the samādhi of non-contention, that I rank foremost among humans, and that I am the foremost arhat free of desire. Yet I do not give rise to such a thought, thinking that I am an arhat free of desire. World-Honored One, if I were to give rise to such a thought, thinking that I have attained the path and fruit of an arhat, then the World-Honored One would not say that Subhūti is one who delights in the practice of araṇā (tranquility without contention). Because Subhūti in reality has nothing whatsoever that he practices, it is said that Subhūti is one who delights in the practice of araṇā.

Subhūti is also known by another name, "Born of Emptiness," for he could constantly perceive the emptiness and stillness of the world. Because he had attained the samādhi of non-contention, he never contended with worldly people: if they said east, it was east; if they said south, it was south; if they said west, it was west; if they said north, it was north. This is because whatever worldly people say is empty—there is no need to contend over who is higher or lower, who is an inch longer or a foot shorter. Subhūti had no desires or thoughts whatsoever toward the world; his mind was empty, without signs, without wishes, and without contrivance. He was the foremost arhat free of desire among humans, yet Subhūti never had the thought "I am an arhat free of desire," much less would he proclaim himself everywhere. He did not cling to the sign of self, nor cling to the sign of "I am an arhat"; otherwise he would be an ordinary person clinging to signs, and the World-Honored One would not have said that Subhūti is one who delights in the practice of tranquility. Because Subhūti's mind was constantly empty and still, without desire or seeking, the World-Honored One praised him as one who delights in the practice of tranquility.

Ordinary beings, by contrast, are the opposite of Subhūti: whether or not there is anything to it, they love to boast about themselves everywhere and promote themselves, love to argue over who is superior and who is inferior, love to show off, love to be pursued and admired by others, love to make themselves stand out—in short, they love to market themselves so that the world knows them, and thereby that self of theirs is gratified. This is the sign of an ordinary being. Apart from the sign of self, ordinary beings can accomplish nothing; they live constantly within the sign of self and the four signs. This is the spiritual sustenance of their entire lives, something they cannot do without. Once that self is suppressed, it is like a fish out of water. In short, whether a person has the Way or not can be seen in actual deeds; one's bodily, verbal, and mental actions are the most revealing, best reflecting a person's level of thought and level of wisdom.