I'm not sure how it is in technical writing, but in software development the top performers are hugely more productive than the average performers. As well, there's a certain degree of risk tolerance: can you trust so and so to go on risky projects?
Even if A and B are doing exactly the same job and producing the product right now, A may be worth more because they can be reassigned to a difficult piece of work, while B can't be for whatever reason.
That said, this may also be a matter of discrimination. I'm sure an employer or three has used this to justify sex discrimination with respect to salary. I do believe that categorical discrimination is intrinsically wrong; is discrimination on the basis of individual characteristics moral?
Marco
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>          Marco Anglesio           |       Whenever books are burned      <
>         mpa at the-wire.com          |    men also in the end are burned.   <
>   http://www.the-wire.com/~mpa    |            --Heinrich Heine          <
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