No idea. But, It reminded me of a story I heard about some kids (I swear not me or any of my friends) who once poured Fertalizer into a curse word on someones lawn. The person saw it the next day and washed it away with a hose and when the grass grew in that spot was always 'greener' than the surrounding grass.
I don't know if it's possible or true, but, maybe certain parts of your soil are more nutrient enriched than other parts?