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Brian McQueen2 big reasons for this that come to mind immediately. One, is that historically, people have been naturally distrustful of cards that are this old being close to mint condition so scammers had to figure out a way of adding wear that's somewhat natural looking to the card. I've never done this and don't know all the techniques but I wouldn't be surprised if baking the card in an oven is actually one of them. I've heard that holding the card up to a candle flame is another.
The second reason is that so many of these reprints come out of the printer or whereever with borders or backs that are noticeably whiter than their legitimate counterparts. If you look closely here, you can see areas on the back of the Lajoie that are much whiter than normal cards this old. The "staining" is added in an attempt to cover the whiteness of the card up and to dull the overall color down to where the "whiteness" doesn't stand out so much.