100% stator ground fault means 100% of the stator winding is protected against ground faults and that includes from the generator terminals all the way to the stator neutral. For 95% stator ground protection means only 95% of the stator winding is protected against ground faults and the remaining 5% which is the neutral part of the generator is not protected against ground faults. Detecting the ground faults at the remaining 5% of the stator winding which is the neutral can be very difficult when a ground fault should occur at that location.
There are schemes that are available to protect the remaining 5% to achieve a 100% ground fault protection to include the measurement of the third harmonic voltage 27THD against the threshold that every generator with active part winding should have, etc.
For 100% protection, low voltage at low frequencies (15 Hz, 20 Hz, etc.) are injected periodically into the stator winding and a measure low frequency current is done at the neutral and that is what predicts if there is a ground fault by measuring the harmonic voltages.
The 100% stator earth fault protection based on injection of a signal is used to detect earth faults on generator stator winding... the big advantage is the possibility to detect earth faults also with generator stopped or unexcited.
The principle was applied with static protection (about 35 years ago) and the selection of the frequency was decided using a sub-harmonic signal (not confused with fundamental or harmonics, especially 3th), easy to reproduce.
BBC (after ABB) decide the use of 12,5 Hz (for a 50 Hz system) and Siemens used 20 Hz signals. The disadvantage is that for Gas turbines starting with static frequency converters, there is a frequency range not protected. Now ABB decide to use an 80 Hz (or similar) signal.