abnormal packet loss in wireshark

I am testing the packETH 1.7 packet genrator and used wireshark 1.2.7 for same. I use two ubuntu 10.04 machine with 100Mb NIC and later added 1Gb NIC.Both machine are connected PtoP with CAT6 straight cable. Now i transmit 200000 udp packet at rate from 125packet/sec to 1000000packet/sec.I used wireshark on both machine to capture packet and measure performance of packet generator. The observation is that in some readings source machine wireshark capture less packets than destination machine wireshark. Why such abnormal packet loss observed on source side when destination machine get same lost packets?