The following waveform shown on the left illustrates a failed overclock. Because of the ground bounce the receiver is no longer able to determine the state of the input. The fix is readily apparent since the error can actually be seen. As displayed in the waveform on the right, shifting VREF slightly upwards, which results in VIL(max) also raising, moves the input signal back into the valid VIL range.
So why not just add some extra GTL Reference voltage to begin with as a safety margin? Because VIH needs to be kept within its operating parameters also. The high level input is subject to the same type of signal degradation as the low level input is.
In the waveform below we have the same low level input as in the above example. The top trace shows the input when it's held high. VIH presents no issue concerning stability as it's within its proper operating range. Even when we correct VREF to make VIL a valid signal VIH remains within its operating range.
But what happens if we take the approach that if some adjustment is good more must be better? As the next figure shows, by increasing VREF beyond what is necessary brings VIL even further into its ideal operating range. But by doing so we have created the same type of out-of-spec condition for VIH that was originally affecting VIL.