Because the nature of the filtering process causes distortions at one or both ends of the data or profile. These distortions are removed to ensure only genuine detail is included in the analysis.
For 2CR filters, data equivalent to 2 sample lengths are discarded from the beginning of the filtered data. For 2CR PC (phase corrected) filters, 1 sample length from each end of the filtered data is discarded.
Because the Gaussian filter has less distorted effect, only half a sample length is lost at each end of the filtered data.
