The commentary for today's reading contains this sentence, "For some unknown reason, the tribes of Judah and Ephriam, and the remaining half-tribe of Manasseh, are given a preferential portion of the land..."  The reason IS known.  Look at pages 92-94 (Genesis 48-49).  Jacob blessed these tribes more than the others. 

Shortly after a general conquest of the land of Canaan, we are told of the land allotment by tribes, all the tribes that is except the tribe of Levi.
Joshua 13:33
But unto the tribe of Levi Moses gave no inheritance; the LORD, the God of Israel, is their inheritance, as He spoke unto them.

The tribe of Levi was given no land allotment. The Lord was their inheritance. Compare that to our lives as believers today.
1Pe 2:9
But you are a chosen generation, a royal priesthood, a holy nation, a people for possession, so that you might speak of the praises of Him who has called you out of darkness into His marvelous light;

What a delightful inheritance (pun intended) we have with Him! We are His royal priesthood, a holy nation, His possession. And what are we to do for Him? Praise Him. He has called us out of darkness of sin and into a life of light.

Caleb was 40 years old when he went in to explore the land of Canaan. He, along with Joshua were the only two of the original multitude (fighting age and above) who were able to go into the land of promise. Why? Because Joshua and Caleb trusted in God’s promises. When Caleb settled in the land he was 85 years old, strong and vigorous.

In the division of the land Zelophehad of the tribe of Manasseh had no sons and so the daughters petitioned Joshua and the leaders for the right to have an inheritance. This will be extremely significant later. In the culture and customs of that day, the women basically counted for nothing, except for having children. Yet if we read the gospels, Jesus certainly elevated women. He had many women followers. No need for us women to be concerned with Women's Lib!

The tribes of Ephraim and Manasseh are numerous and they petition Joshua for more land.  Joshua makes this prophetic statement. 
Joshua 18:17b-18a
"You are numerous and very powerful.  You will have not only one allotment but the forested hill country as well.  Clear it and its farthest limits will be yours..." 
The tribes were given the farthest limits.  This included more than just the land of Israel!  More details when we reach the reading of the book of Jeremiah.