Hi @Jiazhen_Wang,
Looking at the parameters you provided, I think this is just a question of terminology. The “mirror frequency” isn’t the edit OFF frequency, but rather a point mid-way between edit ON and OFF.
Typically for GABA+ (TE=68 MEGA-PRESS) the editing pulses are applied symmetrically to water. So, the edit ON targets the GABA C3 resonance at 1.9 ppm, and edit OFF is the same distance on the opposite side of the 4.7ppm water peak: 4.7 + (4.7 - 1.9) = 7.5 ppm. So here it looks like the Siemens product sequence is specifying the mirror frequency (4.7 ppm, water) from which it derives the symetrically-placed edit OFF frequency (7.5 ppm).
See also: Edit mirror frequency