incorrect and contradictory figures int Netflow Analyzer

Hello.
I'm using NFA 5002, but have noticed the same thing in the 5502
build online demo. The thing is that traffic statistics on different detail level seem to have a very big difference. In our system we have several means of controlling user traffic, including NFA and MS ISA Server log-based report generating software. Considering a period of one month, this software produced a figure of approx. 12 GB of http inbound traffic. When I made a report in NFA on the internet-facing interface and switched to application tab (same period of time), it showed me about 40 GB for http! But the point is that when I click on http and see details on http traffic (IP addresses and so on), it displays total figure of about 12 GB, which goes well with ISA log-based software! I've noticed the same thing in NFA 5502 online demo: for some protocols the overall figure in application tab matches with a figure in the table header for detailed traffic report on that protocol, but for some protocols the difference is just totally confusing. What is the reason and how do I cope with it to produce accurate reports?
That's a big concern of mine and I'm looking forward to hearing some advise or workaround explanation.
Would appreciate any assistance, thank you in advance.