Extremely amazing coincidences, or book plagiarism?

I am researching typography for a book I am writing called Typography for Independent Publishers.

I have about 4,000 books in my personal library and about 100 books are related to publishing. One my favorites is The Non-Designers Design & Type Books, by Robin Williams (no, not the late, hilarious Robin Williams).

Below is part of a scan of one page:

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The highlighted text made sense to me, but it seemed strangely familiar. I took a look at one of my other favorite books about books, Book Design & Production by Pete Masterson.

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Yup -- it's about 95% the same thing.

This seemed really strange. The same sentence appears in two copyrighted books that are sort of competitors. It was strange enough to motivate me to do a Google search, and I found this:

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Yup -- here are those words again, this time in a teaching tool produced by a teacher at a big high school in Texas.

And if that's not enough, I also found the same text on a website operated by the South Newton School Corporation in Indiana. It was apparently copied, but the homepage shows:  "Copyright © 2011 South Newton."

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And, of course there's probably more copycatting out there.

I have no idea who wrote the sentence first, but without attribution the same text can't have multiple valid copyrights. I wonder if the school teachers who have apparently copied the material from another source would approve of a student submitting a term paper with text copied directly from Wikipedia.

Most writers do research. I read lots of book in fields I'm interested in, and try to distill what others have said and then REPHRASE IT IN MY OWN WORDS and try to add my own insights and discoveries.

• With the Internet, if you copy and publish someone else's work you must assume that someone will notice. (It takes big balls to steal intellectual property but it takes a small brain to exhibit the stolen material where millions can see it. I've caught more than 100 copycats of my own work.)
• Back when I was a journalism major at Lehigh I was taught never to copy more than four consecutive words without attribution. That's good advice.
• My own research technique may be imperfect. If I have been an accidental copycat in my 40-plus years of writing, I hereby apologize.