We forgot to include the first gain stage of the common mode servo. The spectra in terms of the input of the servo are actually shown in the attached figure.

Note that the servo for PDHX and PDHY have different values at the first gain stage, +9 dB for PDHX and -9 dB for PDHY. Therefore, the gain for PDHX is about 8 times larger than PDHY. For this reason, the situation is reverted from the previous post; PDHY is noisier than PDHX.

Now it seems that for PDHY peaks seen in PDHX is smeared by the floor noise with weird frequency dependence except for the peak around 20 kHz.