Isn't it obvious?
The money belonged to the late Mr John Smith who had no relatives. The Nigerians arranged by e-mail for Mr David Smith to claim to be John's brother. They have sorted out all the paper work and have paid for modalities. Now all that remains is for them to meet up and split the cash 40/40/20. Come on - deals like this take place 100,000 (ONE HUNDRED THOUSAND) times each day.