Why are there contradictions in the Bible?

Reasons

• Most scholars agree that the OT is a blend a number of slightly different stories that were circulating in Israel when it was first written down, probably between 1500BC and 1000BC. Until that time, the stories had been passed down orally for generations, so it is not surprising that there were variations among different tribal groups.
  
  
  
  

  Wikipedia explanation
  
  For example, the story of Noah's Flood:
  
  The story of Noah and the flood, found in Chapters 6, 7, and 8 of the book of Genesis, is thought to have been composed of two sources referred to as J and P.
  
  The P text here always calls the deity "God" (16 times). The J text always calls the deity by the proper name "YHWH" (10 times).
  
  The P text uses the word "expired." The J text uses the word "died."
  
  In J, it rains for 40 days and nights, and the water recedes for 40 days. In P, the whole process adds up to a calendar year.
  
  In J, Noah releases a dove. In P, he releases a raven.
  
  P has two of each species of animal, a male and a female. J has 14 (seven pairs) of each species of the pure animals (animals that may be sacrificed) and only two of the animals that are not pure. This is important because J ends the story with Noah making a sacrifice—so he needs more than two of each animal or he would make a species extinct!
  
  P has details of cubits, dates, and ages. J does not.
  
  In J, God is personal and involved: known by a personal name ("YHWH"), personally closing the ark, personally smelling Noah's sacrifice, described as "grieved to his heart." In P, God's name is not yet known ("God," in Hebrew Elohim, is not a name; it is what God is), and there are none of the anthropomorphic descriptions that are found in J.
  
  And the point is not just that these differences are maintained consistently in this particular text. These differences are also consistent with the language and characteristics of the other P and J texts throughout the Five Books. P consistently is concerned with dates, ages, and measurements. P uses the word "expired" for death elsewhere (11 times); it never occurs in J. And the distinction regarding the name of God is maintained through over 2,000 occurrences in the Torah with only three exceptions. In the P creation story, God creates a space (firmament) that separates waters that are above it from waters below. The universe in that story is thus a habitable bubble surrounded by water. That same conception is assumed here in the P flood story, in which the "apertures of the skies" and the "fountains of the great deep" are broken up so that the waters flow in. The word "rain" does not occur. The J creation account, on the other hand, has no such conception, and here in the J flood story it just rains.