Differences between Doppler Centroid and frequency in an SLC image in dcEstimate in Oringinal Product Metadata

Hi all,
I am working on a SLC image. I want to evaluate the Doppler Centroid estimations. I found in the User guide of “Sentinel product Specification”, the formula DopplerCentroid=d0+d1(tsr-t0)+d2(tsr-t0)^2. If I use the values of doppler coefficient, slant range times and t0 values in the formula, I find values of Doppler Centroid that are different from the values of frequencies (that are in the fineDcList in Snap with slant range time) estimated. what does this difference mean? and how can I interpret it? Moreover I have to use doppler coefficient of geometry polynomial coefficients or of data polynomial? And if they are three different type of frequencies (doppler centroid geometry, doppler centroid data, frequencies), how can I understand what is good for me?

Thanks in advance
Francesca