I've always suspected it has to do with the acidity of the soil and the soil type. I recently recovered a buffalo that cleaned up like brand new though the date was worn off, other places coins will remain permanently stained and even corroded.
MineLab E-Trac, MineLab Safari, MineLab Explorer XS,
White's 5000 D, White's 6DB, White's 5000 D GEB
Sunray X-1 for the ML's, Garrett PP for the White's and a Backhoe
Fertilizers, insecticides, fumes n smoke from railways...they can all affect coins under the surface once rain washes it in. I've found the same thing in my digs, too. 😊
My pennys are always brown too lol. New drops things come out clean. Find a lot of grandpa's coins someone lost. Even rings are pretty if they are new. City parks must have the worst water as they eat the new coins and zincs pretty quick.