Joshua challenged the seven tribes that had not yet received their portion of the Land of Canaan, and he told them what to do, including having the seven tribes select three men from each tribe, who would survey the land,
And they shall divide it into seven parts. Judah shall remain in their territory on the south, and the house of Joseph shall remain in their territory on the north. You shall therefore survey the land in seven parts and bring the survey here to me, that I may cast lots for you here before the LORD our God (Joshua 18.5, 6).
No apparent significance seems to be attached to the fact that seven tribes still had not taken their portion. As each tribe took their inheritance, the number would countdown and naturally come to seven, then six, and so on.
Therefore, this appears to be an incidental reference to the number seven.