If the Reader would know the reason of this detachment of the Levites, let him consult Joshua 18:7.
Related Commentaries of Numbers 2:33
Numbers 2:33-34
33 But the Levites were not numbered among the children of Israel; as the LORD commanded Moses.
34 And the children of Israel did according to all that the LORD commanded Moses: so they pitched by their standards, and so they set forward, every one after their families, according to the house of their fathers.