I guess that not every person that wotes parts of the bible had full access
to all the previous writings, or if they did, did not totally confirm (had a different idea) previous writings.

This way it is more impressive to have little contradictions, than to wonder about
(naturally acurring) contradictions

Now it is easy to just say, that contradictions are due to the "mortals not fully understanding the
thougts/ways of god".

Even Tolkien did have contradictions in his work, even though this was just one man writing on
this story.