Many modern papers contain developing agents. You can test for this by taking a piece in the light and adding a drop of plain alkali. The presence of an incorporated developer tends to normalize the results in the developing solution you use.
In addition, Phenidone powder developers are more stable than liquid concentrates, and the pH of a final developer or concentrate will affect the rate of hydrolysis of a Phenidone developer.
This subject is rather muddied by the fact that early Phenidone was unstable and meant that Ilford had to delay it's commercial use, later Phenidone's sythesis was iproved making the developing agent quite stable, although not as stable as Phenidone-Z, probably the Phenidone B that Haist refers to. A conseqence is you have statements in books that don't match the facts.
Originally Posted by Rudeofus
The papers I use are extremely responsive to development changes and I do use soft working developers at times so I'd notice shifts with age and they just aren't there, unless a developer has oxisied significantly.