LAN/WAN interfaces show more bandwidth than Queue Tree max limit

I have an issue where my lan/wan interfaces appear to be passing more bandwidth than max limit settings in queue tree.
I have checked that I have a catch all rule for both upload and download in my mangle rules. Does anyone know what might be causing this? My setup is pretty basic. I have also seen this on a default box. When my download is saturated, the queue tree average shows the same as the max limit setting but under interfaces, the bandwidth can fluctuate 3mb/s over the limit.