In his last will and testament, Lehi arranged his posterity into seven different groups, but he went out of his way to divide his property into eight shares. This was because Laman, as the oldest son, was entitled under Deuteronomy 21:17 to a share twice the size of the other sons’ portions of Lehi’s estate. Typically this was the rule because the oldest son had the obligation to take care of his mother and other dependents in the family, and therefore the oldest son needed a larger part of land, or more of the cattle, or whatever the father was dividing up.
But Lehi has a problem. God had called Nephi to be the leader of the people, and Lehi reaffirms Nephi’s call to be his successor. But how can Nephi be Lehi’s successor if he does not have the resources to manage not only his own family but to lead the whole group? And Lehi also wants, I think, to keep balance and harmony in the family as much as possible. So, in order for Nephi and Laman to have equal resources, Lehi gave Laman the "first blessing," while at the same time combining the allocations of Sam and Nephi, putting the two of them into one tribe. Lehi says to Sam, "thou shalt inherit the land like unto thy brother Nephi. And thy seed shall be numbered with his seed" and the two of your will merge. I think this is clever estate planning. Lehi is trying to do everything he can to divide his estate up as fairly and as effectively as he can.
Book of Mormon Central, "Why Did Lehi Divide His People into Seven Tribes? (Jacob 1:13)," KnoWhy 319 (May 29, 2017).