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If I am understanding you correctly, I think the reason it works out is that we're talking about probabilities/proportions, which will always sum to 1. By adjusting the true incidence rate you simply just move the proportioning of false positives/negatives around leaving the same overall 95% accuracy figure.
frankenbike
Absolutely. Agree with all of that.
But the part I'm trying to decipher follows from this: The accuracy of a pregnancy test does not change depending on how many women are currently pregnant.
What does change is the raw number of positive and negative results due to there being more people who can get false negatives. But any given man or woman will still have the same likelihood of getting a particular result.