Need some help: NFA working incorrectly

Hi, guys!

I use NFA6 to monitore my vpn-router Cisco 831.
Here's commands used to configure router:

Conf t
interface Ethernet 0
ip route-cache flow
exit
ip flow-export destination 192.168.45.3 9996
ip flow-export source Ethernet 0
ip flow-export version 5
ip flow-cache timeout active 5
ip flow-cache timeout inactive 15
snmp-server ifindex persist
do write

where:
192.168.45.3 is NFA6-host on Windows XP Pro SP2
Ethernet0 is INTERNAL interface of router

After 2 weeks of working I've got results:
NFA6: 1.98Gb
ISP: 6.5Gb

ISP billing works correctly - I sure.

Today I've reconfigured Cisco this way:

Conf t
interface Ethernet 1
ip route-cache flow
exit
ip flow-export destination 192.168.45.3 9996
ip flow-export source Ethernet 1
ip flow-export version 5
ip flow-cache timeout active 5
ip flow-cache timeout inactive 15
snmp-server ifindex persist
do write

Ethernet 1 - external interface.

In main windows of NFA has appeared 1 new interface. In some time (when statictics has been collected) I looked at data: Hour report showed data mismatch between first interface and second. Traffic from Ethernet 1 was like truth but from Ethernet 0 wasn't i.e. smaller that fisrt one.

What the way of configuring router is more correct? Maybe not one of these?