Please understand, I'm no chemist; I studiously avoided it in school...but here's my version of what happens with plain D-76 from people who I believe know.(posted 8359 days ago)At the freshly-mixed pH the hydroquinone serves to regenerate the metol but has virtually no reducing action itself. As D-76 ages, the pH rises and the hydroquinone becomes more active, thus causing an increase in resulting neg contrast.
For that reason, the packaged D-76 Kodak sells in the US is a buffered version that has about half the buffering of D-76d.
Grant Haist's solution was to simply omit the hydroquinone and increase the metol, giving D-76H about the same activity as D-76. My speculation is that although the pH also increases usually oxidation counters its effect and since there's no hydroquinone to "activate" the pH rise doesn't matter.