I get the same. I figured this is the 16.5kw, just that we have less voltage drop to the charge point, right?
Anyway, I charge in two locations. One on this and the other on single phase 32a commando. In a house I expect to own long term I'd pay a fair bit for the combined privilege of faster charging and leaving the UMC rolled up in the car. Overnight makes no odds but any need for say 2 hours of charging and the difference in convenience can be quite a bit.
You also have just shy of 250v at that point in time (it will fluctuate) which will be upping the power considerably. You are drawing a calculated 72A (24A/phase which correlates to the display) so reworking that to nominal 230v, thats the quoted 16.5kWh - power is a factor of voltage, so the higher the voltage, the more power for same current/amps.
I'm guessing there is a PV solar (or other microgeneration system) install near by? A bright April/May day with occasional clouds and a cooling northerly does wonders for peak power PV generation. Probably a higher voltage too (not measured) experienced in these conditions would have kept average power high for same 16A current limit too. Days energy generated, 17.8kWh (11th) vs 22.7kWh (9th).
You will only get 18kW if the voltage is high (and the number on the display is rounded, so it might be 17.6kW).
The standards for EV chargepoints call for the available capacity to be communicated to the car only as a maximum current; power = voltage * current, so if the car takes the maximum current it is permitted then the power will vary with the voltage. Advertised power ratings usually assume nominal voltage (230V). If the capacity of the chargepoint is greater than that of the car, then the car applies whatever limit the designer considers appropriate (could be current, power, temperature etc.), but Tesla's charger design is also primarily current limited, so for example on a "43kW" chargepoint (3-phase, 63A, nominal 230V/phase), a Tesla will draw its maximum current (16A/24A/32A according to model) and the exact power that results will again vary with the voltage (which is not typically under the user's control).
The older Model S chargers are definitely limited purely by current - they are rated up to 277V (and used at that voltage in Superchargers, giving significantly more power) and always take the fixed 16A/32A unless limited by other factors (like the battery being full). There were reports from the USA that the newer S/X chargers additionally have an overall power limit - such that as the voltage increases, the power goes up until it reaches the power limit, then with further increase in voltage the car reduces the current to keep within the power limit. I have not seen any such reports from over here; possibly the design is different, or possibly we never get up to voltages high enough to see the limit.
Quite a lot of Ecotricity sites do show relatively high voltage, as they've deliberately sited them very close to the building's electrical supply (to minimise cable length and so their install cost), and many of these MSA sites will have dedicated supply transformers.
Solar is a common explanation for high voltage on domestic supplies, but you will see high voltage whenever you are physically close to the transformer. Common practice is to set transformers to deliver the maximum permitted voltage (253V) with no load, and the voltage then droops the further away you are and/or the higher the load.