It Depends Who You Are Talking To

במדבר לא, כא: וַיֹאמֶׁר אֶׁלְעָזָר הַכֹהֵּן
Baal HaTurim explains that Elazar taught them about the laws of hag'alas keilim (the kashering of vessels from the remnant of non-kosher food, the subject of verses 21-24), whereas in the previous parshah, Moshe taught them the laws of purification from tum'as meis (see Rashi pasuk 21). This is because when Moshe spoke, it was to the captains who brought back precious garments and the like (which needed to be purified); Elazar on the other hand, was speaking to the soldiers at large, who came back with household vessels which required hag'alah.