The Whole Congregation Assembled To Do the Will of Yahweh (Numbers 8:9).
“ And you shall present the Levites before the tent of meeting. And you shall assemble the whole congregation of the children of Israel,”
This is not in chronological order, but introduces the gathering of the people where it is convenient. Much would be taking place at the same time. The people would thus have already gathered first prior to the presentation. The writer's point is that they had been gathered prior to the ceremony. It is the presenting of the Levites ‘before the Tent of meeting', as an official presentation, that follows chronologically after the taking of the ox bulls, the former at least following the assembling of the congregation. As always chronology is unimportant.