They are not. The bottom 50% of people owns a fraction much smaller than 1/2 of all available wealth.
What Oxfam is saying is: if you rank people by wealth on the x axis then plot their wealth of the y axis, the integral (sum, or area under the curve) of the function on the first half of the graph is equal to the integral on the last 85 points. Try actually drawing it, it might help you understand.
What Oxfam is saying is: if you rank people by wealth on the x axis then plot their wealth of the y axis, the integral (sum, or area under the curve) of the function on the first half of the graph is equal to the integral on the last 85 points. Try actually drawing it, it might help you understand.