good point, sorry Ark. it does look like that. Perhaps just a good rinse and then another treatment?  if residue was left behind, it would cut down on the surface area ratio and cause uneven bleaching.  
Here is a thought however to support that:
Plastics are hydrocarbon polymers ->  This means that they attract things like fats and oils - (soaps, or any oils) If they would have gotten on the shell would be absorbed into the plastic - blocking the sites where the oxidation would occur. 
I do hope you can save that famicom Dan.