Why does the Torah see fit to inform us that Aharon and his sons did everything that Hashem commanded Moshe?
Rashi: To teach us their praise in that they did not deviate one iota from what Moshe had instructed them.
See Sifsei Chachamim, who lists the many places where the Torah writes this phrase and explains why it does not do so in connection with the Korban Pesach (See Bamidbar 9:5).
Why does the Torah deviate from the usual expression "Ka'asher Tzivah Hashem Es Moshe"?
Ramban, Moshav Zekenim: Because they did not just do what Hashem commanded, but added to it, by bringing 'a strange fire' - which Hashem did not command them to bring
As we will see below (10:1).