What Is The Mystery In Agatha Cristie's Novel Why Didn't They Ask Evans?

Agatha christie is a great mystery novelist and i have read her book why didnt they ask evans. The story starts with two people playing golf and when the golf ball goes over the cliff the two discover that there is a man lying down there. one of them calls the emergency and one whose name is Bobby bends down to check if he is alive. The man's last words were why didnt they ask evans? at that time he doesnt pay much importance and even when he is asked to  appear in court he does not think it is  something of importance to disclose.

It is when he tells the dead man's alleged sister the details even though she is not really his sister, he realises that there is more to his death then it appears. it turns out that the dead man was solving a mystery of his dead friend who left a will. Instead of asking the maid whose name was evans , the gardener and the other  worker were asked to testify. So this is something that he wondered that why didnt they ask evans? the reason is what you have to read the book for