Sometimes it can be confusing and rather hard to understand why there are apparent (not real) contradictions in the Bible. In Ben Witherington's recent book he comments on one of these that I thought might be of help.
In the parallel accounts of 1-2 Samuel and 1-2 Chronicles, there are some apparent problems. For instance, what did Nathan actually say to David? What 2 Samuel reports or what 2 Chronicles reports? A little of both? Neither? At this point, Witherington points out that the principle of subsequence is pretty important as one tries to historically account for the diversity of these two accounts. In his words:
"First and second Samuel was written long before 1-2 Chronicles, and the latter text
presupposes that the former is in play and widely known, which is precisely what allows the
Chronicler to take some liberties in order to make his own theological and ethical points. He
assumes a preexisting fixed text."
It's good to keep some of these things in mind as we read through the entirety of the Bible.