You haven't told anything about your configuration, so this is just a guess (from the online help):
"The accounting background process will sample data from the interfaces specified here. In normal setups, you should only collect data on ONE interface, since forwarded packets will be recorded on each interface they pass.
If you use masquerading, it is recommended to collect data on your internal interface, since the packets leaving the firewall on the external interface are already re-written with the new source address.
You can set a list of networks to ignore in the accounting process. Traffic to and from the networks listed will not be accounted at all. "
