The mm accuracy is in theory possible and can be sometimes verified using a corner reflector or a well-defined permanent scatterer (i.e.. phase center always at the same position) monitored with precise GNSS measurements. In reality, InSAR data need to be average to reduce the impact of different noise terms and signal decorrelation plus some statistical estimation of the surface movements when computing the surface deformations. The mm accuracy is what is possible to achieve but the accuracy of some persistent scatterers can be in the cm range. This is why it is necessary to use large stacks of data of at least 15-20 images for PSI.