I think I answered you a little quickly and I must clarify in relation to your remark:
"the contracted speeds correspond to the theoretical maximum (e.g. up to X Mbits/s or Y Gbits/s)
it must be taken into account that part of the traffic is reserved for technical or system data"
- When we talk about bandwidth or Mbits/s it is the entirety of the bits which pass through the Wifi and not as you imply the significant bits, consequently the technical data, as you call them, are included in measuring WiFi traffic
- Measurement software such as those cited does not analyze the content, it simply counts the number of bits passing each second, it does not benefit from the intelligence which would allow them to dissociate useful information from technical information. They only see binary values 0 or 1
Consequently the 60% measured must be able to be further improved to move towards 100% probably without ever getting there!
A+