Not to rain on the parade, but its a plausible explanation. Occasionally the method of printing the reserve notes is figured out by someone. In the past, entire presses have been stolen, or their ink duplicated. Its very possible this is an anti-counterfeit measure. Of course, you would say the whole concept is counterfeit, but you know what I mean. Counterfeiters do things like order a few dollars worth of food from McDonalds and then get the change back. Before they are discovered when the draw is counted, they are gone.
Now, with that being said, I imagine when the old notes leave circulation they will be destroyed (and, of course, even further replaced with new ones). This further makes it seem like anti-counterfeit. Also, while the Treasury Secretary is on the left, he is also on the right of Franklin. Once most old bills are out of circulation, anyone who sees an old one will be more likely to be suspect to its origins. Imagine presenting money in denominations from a hundred years ago at the grocery store. Well, you get the picture.
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