Linux: Interface statistics missing packets?

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http://serverfault.com – I have a server with a network interface connected to a switch port that is configured to mirror all the traffic that goes through the switch, which means that if I run tcpdump on that interface I see a lot of packets. The strange thing is that all these packets don't show up in the interface statistics, neither as "received" nor as "dropped". Why aren't they being counted? (HowTos)