coherence is a measure for the quality of the interferometric information. If coherence is high (just like in major parts of your image), you can more or less trust the unwrapped interferogram to be correct. Areas of low coherence (the black ones) are decorrelated due to changes in landcover or atmospheric distortions.
You can define a valid pixel expression for the InSAR DEM in the band properties (coherence > 0.3) to mask out all low coherence areas from the scatterplot. The result should then be clearer in terms of relation to the SRTM.
This discussion might be helpful to you. It deals with Differential InSAR but the principle of masking out low coherence areas is the same: Subsidence map in 3d view