@Carl Love I see your point for pdf1 and pdf2 which are procedures. As written, they always return a numeric value no matter the argument. (Why do they do this, and what is the magic value they return?)

But for pdf3 and pdf4, a symbolic answer is returned. I just noticed that if I evalf to 200 digits, pdf3 doesn't have any error, so I'll attribute that to precision gremlins. But your explanation doesn't seem to explain why there is a discrepancy with pdf4.