Total Bytes calculation vs SNMP tools

Hello,
I am using NFA in a router that is polled by another tool.
NFA is reporting for Netflows informations transmitted by the router (Cisco) in the WAN interface (ETHERNET).
The other tool is collecting data every 5 minutes on the same interface.
Excluding 5 minutes polling vs continous flow issues, I am comparing daily volumes on both tool with BYTES IN and OUT for a day.
The SNMP tool calculate 36.3 GB IN and 6.5 GB OUT. NFA reports for 34.8 GB IN and 5.7 GB OUT.

Can you tell me how the NFA is calculating the Volume DATA ?
Is it taking care for TCP-IP-Ethernet headers or just payload ?

Dis you experiment already this kind of questions?

Thanks in advance...
...Francisco.