When you collect data at the Zinc K-edge, you will have a very small component of Co in the f'', since your protein as both Zn and Co.
On the other hand, the f' will still have a significant contribution of both Zn and Co.
I'm confused, isn't this backwards? Based on the plot, Co f'' will still be nearly 3e- at the Zn K-edge, but the f' will be close to zero.