I'm struggling a bit with the two config options for RelayBandwidthRate and RelayBandwidthBurst - what I currently have in my torrc config file is:

...
RelayBandwidthRate  8 Mbit
RelayBandwidthBurst 12 Mbit
...

My assumption was that on average the Relay would consume 8 Mbps max. but over a shorter period of time also up to 12 Mbps in case needed.
What I see (historical monitor data at 10 s intervals for the past 6 months) is that the bandwidth usage never exceeds 8 Mbps. I can also see the consumption to be levelled out at 8 Mbps for minutes or hours so it looks like there would be a need for more bandwidth.
My question: why can't I see the bandwidth usage going above 8 Mbps and making use of the allowed burst? Btw, I'm typically using the latest stable tor, currently the relay is on 0.4.0.5.

Any comment welcome!