It's because the net concentration of hydrogen ions in solution didn't change...
What measuring water pH doesn't tell you is it's alkalinity. It measures hydrogen, not bicarbonates, or carbonates.
Bicarbonates and carbonates in your irrigation water are what will affect affect soil pH over time. Citric acid (what you're using) is a "weak" acid. The purpose of adding citric acid to your irrigation water water isn't to lower the pH (hydrogen ion concentration in solution), it's to neutralize the alkalinity (bicarbonates). While there is a temporary drop in pH after adding it, as it reacts with and neutralizes the bicarbonates in your irrigation water, the "acid/hydrogen" concentration that it temporarily added to the solution is neutralized in the reaction as well.
The alkalinity in your water has been neutralized, but the hydrogen ion concentration in your water hasn't changed (much). The fact that the up drift pH reading is lower than when you started demonstrates that you've effectively neutralized all of the alkalinity in your water (there's "extra" acid in the water, as the reading is lower). I wouldn't worry too much about the pH now...
