Grove PH sensor


I looked at the schematics for the Grove pH probe and could not understand one part, maybe I misunderstood something. The OPA333 dual-opamp is connected on single supply (V- to GND and V+ to 3.3v) and the pH sensor reference is also tied to GND. I can understand how it buffers and amplifies positive voltages (pH 0 to 7) but once the probe hits the negative rail the opamp should flat it out to GND, how can it measure pH 7-14?