In a recent Nature Nanotechnology paper (attached) titled "Single-molecule diodes with high rectification ratios through environmental control", how does the asymetric electrode change the rectifying behavior of the molecule?

In the supplemental info (attached, Figure 5) the authors include a CV of ferrocene and claim that this is further evidence of the asymmetry. What is the logic used here? 

http://www.nature.com/nnano/journal/v10/n6/full/nnano.2015.97.html