This is a ratio of PD1_I to PD1_Q (so a ratio of the two quadratures of AS166), measured in an anti-spring state. It's not flat because our set up has single sideband RF heterodyne detection, and using a single RF sideband as a local oscillator allows one to detect different quadratures by using different RF demodulation phases. So the variation in frequency is actually a measure of how the frequency response of DARM changes when you vary the detection quadrature. This measure is imperfect because it doesn't account for the effect of the DARM loop.
Even though you can choose your detection quadrature with this setup, you can't get squeezed quantum noise with a single RF sideband. The quantum noise around the other (zero-amplitude) RF sideband still gets mixed in, and negates any squeezing benefits.