(19:2) This is a statute of the Torah which Hashem commanded, saying: Speak to the children of Yisrael and they shall take for you a perfectly unblemished cow upon which no yoke was laid.

We find many mitzvos in the Torah whose reasoning is not explained and thus are also statutes, yet the Torah does not refer to them as such. So why is the mitzvah of the red heifer different?

The answer is that although Chazal taught that this mitzvah was to atone for the sin of the golden calf – “let the mother come and clean up the mess made by the child” – Chazal also taught about the verse in Shemos 15:25 “there He made for them a statute and a judgement” that they were commanded in the mitzvah of the red heifer whilst they were in Marah, and at that time they had not yet sinned with the golden calf.

Thus this was not the reason for the mitzvah of the red heifer, and therefore the Torah called it a statute.