tcpdump - packets dropped by interface

The man for tcpdump states what can cause "packets dropped by kernel" but it doesn't state what can cause "packets dropped by interface". Can anyone shed some light as to why the interface may be dropping packets? Or how I can find out the reason for it dropping the packets?

I'm using Ubuntu 14.04 via Virtual Box on a Win 7 host. The NIC is a USB to Ethernet adapter.

TIA