You asked what the difference was, and I said the difference. Was it unclear that to fit the phrasing of your question, we add "OCR doesn't"? I would not personally call Jbig2 OCR.
reply> You asked what the difference was, and I said the difference.
replyTake another look at my comment.
Let me try rephrasing to make the response to your original comment as clear as possible.
replyQuestion: "How can we describe OCR that wouldn't match this definition exactly?"
Answer: This definition largely fits OCR, but "reference to a single instance" is a weird way to phrase it. A better definition of OCR would include how it uses builtin knowledge of glyphs and text structure, unlike JBIG2 which looks for examples dynamically. And that difference in technique gives you a significant difference in the end results.
Is that better?
The definition you quoted is not an "exact" fit to OCR, it's a mildly misleading fit to OCR, and clearing up the misleading part makes it no longer fit both.