Why are the fasted days described as "a perfect ten"?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. It means they are complete in serving as a substitute for the sacrificial animal. [Ibn ʿAbbās, al-Ḥasan] Qāḍī Abū Yaʿlā said: "One might have thought thought that the three days alone suffice as a replacement for the sacrificial animal in terms of reward, so Allah informed us that the full ten days are what serve as the substitute."
2. The conjunction "and" can sometimes mean "or" in some contexts. For example, in the verse: "Then marry those that please you of [other] women, two or three or four". Allah removed any possibility of choice in this verse by saying: "Those are ten complete." [Al-Zajjāj]
3. It serves as an emphasis. The Qurʾān was revealed in the language of the Arabs, who are in the habit of repeating things for emphasis.
4. It means: "Those are ten complete in virtue," even though three days are to be observed during Hajj while the seven are afterward. This is to prevent someone from mistakenly thinking that the seven are inferior to the three. [Abū Sulaymān al-Dimashqī]
5. It is a declarative statement but implies a command, meaning: "Those are ten, so complete them."