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Only Rod can answer that. I could make a practical guess, original spring started out with 21 interfaces. and implementation got bigger over the period. All I remember reading once, Rod started out with a small framework for the place he was working at and after that he realized how nice it was and how big a room it has to evolve to a full blown j2ee framework. I am not sure if idea came from Brad Appleton's Pizza Inversion.
First: I do not know.
That said, I would also like to make a guess
I think the code base had been packaged using Rod's company name (which is named Interface21). As far as I recall the framework was just code accompanying his book and was afaik never referred to by a specific name.
As of the name itself I would guess that it might mean something like "interface to the 21st century" or so. Just an idea, though.