How does Wirehark calculate the clock drift for RTP?

Hi,

I made an rtp capture and obtained the following statistics :

Max delta = 26,74 ms at packet no. 482 
Max jitter = 0,94 ms. Mean jitter = 0,38 ms.
Max skew = -6,65 ms.
Total RTP packets = 92   (expected 92)   Lost RTP packets = 0 (0,00%)   Sequence errors = 0 
Duration 8,20 s (-1842 ms clock drift, corresponding to 6203 Hz (-22,47%)

I do not understand how it calculates -1842 ms clock drift since in the trace there is no difference between the timestamp of wireshark and the timestamp of the RTP stream. Both shows 8200 ms (8,20 s).

regards Bruno Scornet