There are two possible reasons: Either you have a hub that is only labeled as a hub, but inside is a switch (some vendors like Linksys do that), or you have a multi-speed hub, in which case you can't see the traffic from the stations operating at the speed that is different from your NIC's speed (e.g. if you have a 10 Mbit NIC, you can't see the traffic generated by 100 Mbit NICs).
