tshark reporting packet loss

Greetings All;

I run tshark to capture date for 45 seconds. At the end of 45 seconds tshark stops and reports:

6504 79 packets dropped

I assume that 6504 is the number of packets captured.

I am confused as to the 79 packets dropped. Are these packets that are somehow dropped by wireshark (maybe for memory reasons ?), or are these packets it believes are somehow dropped in the network.

I am running virtual box guest on a win 7 host, and running wireshark within the guest. The guest is in "bridged mode" so it has its own network ip. Is there anything I need to be real careful about when doing this ??

thanks as always; Walter