Corrosion of steel nails: why are the anodes and cathodes where the are?

Hi, i am doing a corrosion experiment for chemistry and i'm using phenolphthalein and potassium hexocyanoferrate indicators to see where oxidation and reduction occur on the nails, which are set in agar jelly in petri dishes. i have set up 9 petri dishes and there doesn't seem to be a clear pattern as to where oxidation and reduction are occurring, as indicated by the phenolphthalein turning pink at the cathode (reduction) with hydroxide ions being formed, and the potassium hexocyanoferrate turning blue at the anode (oxidation) with Fe2+ ions being formed. So where the anodes and cathodes are on the nails in the 9 petri dishes are all in different places, so I was just wondering if oxidation/reduction are occurring on random places on the nail (the nails haven't been stressed/damaged at all) or if there is some logical explanation for what's happening or what should happen? Thank you for your help!!