I have to agree with Ian on your observations. In fact, you may well have your findings exactly opposite of what is generally known about developer characteristics.
Originally Posted by Gerald C Koch
Pyro based developers harden the gelatin of the film during the very first stages of reduction, grain is set at that point with relatively little change to the grain's accutance or appearance no matter the length of exposure to chemistry.
Whereas, developers with a significant presence of Sodium Sulfite do in fact promote silver migration during the last stages of exposure to chemistry. Such migration and lack of stability directly go to the accutance and impression of sharpness in the final negative / print.
MSDS information on Xtol shows a moderate amount of Sodium Sulfite makes up the formula for Xtol.
Lastly, I have never looked through a microscope at any negative, I have however seen tests done on Tri X & FP 4 developed with HC 110 and the Pyrocat HD. THese tests were conducted by a respected moderator on the LF Forum and bare out with striking clarity the thoughts I shared above.