Sometimes it is hard to say. From the data sheet.
bit 0 GPIF: Port Change Interrupt Flag bit
1 = When at least one of the GP5:GP0 pins changed state (must be cleared in software)
0 = None of the GP5:GP0 pins have changed state
If one of the two buttons were down and the other up when the bit was cleared,( made zero), then releasing one OR pushing the other will cause the flag to be set, (bit 0 = 1) , just looking for a change, any change.