I agree this should be consistent for the backplane interfaces across the platforms, however, there are a lot of ways the "total" throughput has issues.
We have firewalls with;
bonded interfaces
bonded interfaces with VLAN's
individual interfaces with no VLAN's
and individual interfaces with VLANS.
All of those can result in doubling or tripling of the "Total" throughput as the data is counted on the physical interface, the bond and the VLAN. Some firewalls we have a mix of these interfaces on the same device.
So until some intelligence is built into the math of the total's, which is possible to do, the total is not a super helpful item to look at.
Right now I've just taken to looking at a stacked graph of all the throughput of all the interfaces and I turn off manually anything that is doubled if I really want to know what is going on. If someone comes up with a good "IF" statement to make the total throughput better, let me know.