Commentaire sur La Genèse 7:9

שנים שנים TWO AND TWO —In this number they were all equal — the least number was two.

שנים שנים באו אל נח. They came to Noach in pairs. The Torah points out that these animals came of their own volition; Noach did not have to search for them or catch them. One of the reasons the Torah does not mention seven pairs of pure animals here is that only the number of animals that came to the ark to maintain their species are mentioned here. G'd guided them to the ark and Noach admitted them. The additional animals that served as תשלום, payment, Noach had to provide himself. You find this confirmed in verse 15 when the animals that came to the ark are specifically described as being only pairs.

שנים שנים, in pairs. Seeing that a pair is made up of a male and a female, two pairs of each species came. כאשר צוה אלוקים את נח, the animals came of their own accord, but Noach brought them into the ark as G’d had commanded him to do.