Perhaps "the edges" (fields around the perimeter) are protected by the lettering when the coins are both handled & cleaned.
Just like all coins, when in circulation areas in between busy details never get touched and dirt can accumulate there, as will the last traces of original luster. The high points and clear fields get both dirt as well as original luster quickly worn off. Its pretty much a standard thing a collector should understand when evaluating coins Tim. This little factoid will enable you to spot whizzing, cleaning, and lots of other things. Its good to thoroughly understand this point. If your coin posted DIDN'T have these attributes, I would assume it was cleaned.
Canadian large cents don't turn dark towards the edges. What you have is a chemical reation there where it has been handled or in a holder. PVC corrosion starts to turn things dark if it doesn't stay green. Acetone SOMETIMES will take it off. Look through a loupe and see if it's all on the surface ..if so acetone will take it off ..but don't sniff it!
Oxidation occurs over time, plus grease-n-grime builds up within the finnicky detailed areas ... but circulation-wear will keep the "open areas" and/or the "high-spots" far more clean ... therefore, the coin begins to look a bit two toned!! => here is a pretty drastic example of a nice, ol' cool silver coin ...
