Why do Atheist seem to know more about the bible than Christian ? Have they something to prove ?

I think there are a couple reasons for that:

1) Atheists who began as theists questioned what they were being taught by the church and read for themselves the book upon which the teaching is founded and in doing so discovered that certain associated beliefs were nonsensical, contradictory or absurd.

2) Atheists who were never theists either made an earnest attempt to expose themselves to the source material or went looking for justification for their dismissal.
In either case, the average Christian knows what they know of the Bible only what they've been told from the pulpit or Sunday school and that mainly from the New Testament while many atheists have actually taken the time to read the Bible cover to cover.