I agree with smiddy3
An unbiased reading would show that writer of Genesis is struggling to create details, and in his desperation repeating the details already written in small scale into large scale later:
1) From the promise given to Abraham earlier (“I will make your offspring like the dust of the earth, so that if anyone could count the dust, then your offspring could be counted”—Gen 13:16) it is obvious that children would be born through Abraham. Yet we find all unnecessary scenes of Sarah asking Abraham to have child through their maid-servant: Hagar, and then dismissing her and her child later following family problems.
Later (Geneses 29-31) we find the same drama in larger scale: God is poorly depicted as fueling the competition of Lean and Rachel in giving birth children, and in the process giving the maidservants to their combined husband: Jacob
2) Abraham is brought out of Chaldeans to “take possession of land” (Genesis 15:7)—a small scale incident.
Israel is brought out of Egypt to “take possession of land”—a large scale event.