Why would one coffee taste much sharper than another while both have the same pH?

Answer: Because it contains more acid overall

Because it was served at a lower temperature — Serving temperature can change how your tongue reads acidity, and colder coffee can mute sweetness, making sour notes stand out in context. But temperature does not alter the coffee's total acid content. If the two cups have the same pH, their free hydrogen ion activity is the same; the sharpness difference is better explained by the total buffered acid load, not by coldness.

Because it contains more acid overall ✓ — Yes – pH records only the concentration of free hydrogen ions at a moment, not the total amount of acid in the brew. A coffee can have the same pH as another yet carry significantly more total acid (also called titratable acidity), and that total acid quantity is what your tongue experiences as sharpness. Acids differ in how much they dissociate, so two cups with matching pH can still differ in their overall acid load.

Because it was roasted darker — Roast color does alter flavor: darker roasts tend to be more bitter and less bright because some acids break down during roasting. But after two cups are brewed, the same pH does not guarantee equal total acid content; the difference in sharpness comes from the total acid in the cup, not from the bean's roast level. Roasting alone cannot make that sharper taste when the hydrogen ion activity is already equal.