Rub two Pennies together?

[Digger54]

With heat advisory going on I relegated myself to organizing and putting away a bunch of wheat pennies my daughter gave me that she found during a post Katrina clean up. One of the coins was pretty grungy but I still knew right away that it was a steel penny. When I cleaned it up I discovered that one side had traces of copper on the surface. The only explanation I can think of is that at one point that side had been rubbed back and forth on another wheat penny and copper being softer than steel became deposited on the coin's back side. Any other theories?

Here is the penny next to another steel penny...

 

[l.cutler]

I would say more likely it is one of the copper plated steel cents sold as novelties to mimic the rare copper 1943 that the copper has worn mostly off of.