For tap water with lots of calcium, phosphoric acid based pH down sometimes does a bad job at lowering the pH. This is because calcium phosphate is very insoluble in water. When phosphate and calcium ion levels are both high, and the pH gets too high, calcium phosphate will precipitate out of the solution, then all bets on pH are off.
Since calcium nitrate is very soluble in water, when you add nitric acid to tap water with lots of calcium carbonate, the nitrates from the nitric acid can not form precipitates with the calcium.
Products like ionic have a hard water version that replaces some of their calcium nitrate with nitric acid. You end up with the same thing because the calcium carbonate from the tap mixes with the nitric acid giving you calcium nitrate and CO2 gas.