Live traffic not showing correct values

We're testing a demo of the product. We're using a netscreen NS208 with 8 interfaces. When we look at the live traffic data it shows that we are only using 1.5MB inbound, outbound is even less.

However the MRTG graph for our untrust interface shows our internet connection is buried at 4.5MB.

We checked to make sure we weren't confusing inbound with outbound. In short, we expected to see the 4.5MB internet traffic PLUS all the traffic going between the interfaces on the netscreen.

Any suggestions as to why the numbers are off? If we can resolve the first issue, is there a way to only monitor the untrust interface?

Thanks.