kalibrate-rtl results...

I'm attempting to calibrate my RTL dongles. I notice they are all about 2-4khz off if I compare against a known broadcast station's frequency. I am trying out kalibrate-rtl and am getting varied results. I am in the US, so I did a 'kal -s EGSM -g 20' and got the following: chan: 991 (928.4MHz + 26.986kHz) power: 448020.99 chan: 1002 (930.6MHz - 5.631kHz) power: 320357.07 chan: 1003 (930.8MHz + 16.192kHz) power: 109489.92 Then 'kal -c CHANNELNUM -g 20' for each channel number. I then get the results below which the average frequency error looks different for each channel. It is consistent though if I run them multiple times. I would have expected less frequency error since I know that it is really only about 4Khz off. Or am I not reading the results correctly? -Rick Using device 0: Generic RTL2832U OEM Found Fitipower FC0013 tuner Exact sample rate is: 270833.002142 Hz Setting gain: 100.0 dB kal: Calculating clock frequency offset. Using E-GSM-900 channel 991 (928.4MHz) average [min, max] (range, stddev) + 21.342kHz [17837, 27433] (9596, 3402.206299) overruns: 0 not found: 199 average absolute error: -22.988 ppm Using device 0: Generic RTL2832U OEM Found Fitipower FC0013 tuner Exact sample rate is: 270833.002142 Hz Setting gain: 100.0 dB kal: Calculating clock frequency offset. Using E-GSM-900 channel 1002 (930.6MHz) average [min, max] (range, stddev) - 11.529kHz [-15293, -5679] (9614, 3640.780273) overruns: 0 not found: 194 average absolute error: 12.389 ppm Using device 0: Generic RTL2832U OEM Found Fitipower FC0013 tuner Exact sample rate is: 270833.002142 Hz Setting gain: 100.0 dB kal: Calculating clock frequency offset. Using E-GSM-900 channel 1003 (930.8MHz) average [min, max] (range, stddev) + 14.283kHz [9462, 19098] (9636, 4777.044922) overruns: 0 not found: 126 average absolute error: -15.345 ppm