mod cband+mod proxy

[bb22]

So i have a web server with a single virtual host.
It is a reverse proxy that accesses a number of backend http servers. It also uses mod_cache to store static files locally and serve them directly instead of request them to the backend servers. Also some files are served directly from the local filesystem (and not from a cache).

I did 1 hour of testing :
My hosts' mrtg graphs (based off router traffic) say my total outgoing bandwidth usage is about 30Mbps (ie:3.75MBps) over that period.
Apache's mod_status says:
- Total Traffic: 18.5 GB
- 120 requests/sec - 3.3 MB/second (average values over that period)
Which sounds OK with mrtg.

But mod_cband reports :
Total traffic for virtual host: 5.29GiB
Reported speed is in general between 2000 and 4000 kbps (250-500kBps)

Why such a difference? What I suspect is that mod_cband is currently only measuring the local traffic. I'm not even sure it measures the mod_cache traffic, and I'm almost positive it doesn't measure the proxy traffic.
Does anyone know how to make sure mod_cband is active, whatever way the request is being treated (mod_cache, mod_proxy or local request)?