Why can we not compare the coupling constants to chemically equivalent (but not identical) protons to determine magnetic equivalence?

I have been learning about magnetic inequivalence today, and I find it quite hard to undersand. See, for example:


[![para-nitrophenol](https://i.stack.imgur.com/81Vrw.png)](https://i.stack.imgur.com/81Vrw.png)


In this case, according to the book I am using, $\ce{H\_\mathrm{a}}$ and $\ce{H\_\mathrm{b}}$ may be chemical equivalents, but they are not magnetic equivalents, due to the fact that $J\_\mathrm{{\ce{a-c}}}$ and $J\_\mathrm{{\ce{b-c}}}$ are different. Which I understand, but should we not consider that there is a $J\_\mathrm{{\ce{b-d}}}$ which is equal to $J\_\mathrm{{\ce{a-c}}}$?
I do not understand why they are magnetically different, but both have to chemical equivalent protons, one in *ortho* and one in *para* position. 




If, as you say, you understand that Ja-c is different to Jb-c, then you should be able to rationalise that A is different to B. Therefore A will see B as being different to itself and there will be observable coupling between the two. That is the crux of magnetic non-equivalence; not so much as having different coupling to other nuclei around them, but being different to each other and therefore manifesting coupling between themselves.


As always, I'd recommend reading [Reich's online NMR resource](http://www.chem.wisc.edu/areas/reich/chem605/) (symmetry section) to learn more. 


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The problem is that from the perspective of $\ce{H\_{d}}$ and $\ce{H\_{c}}$, $\ce{H\_{a}}$ and $\ce{H\_{b}}$ are different (you can tell which is which by their coupling constants). This difference means that there is a magnetic inequivalence between the two hydrogens $\ce{H\_{a}}$ and $\ce{H\_{b}}$, so there will be coupling between them.


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