Eamonn

Not really. What the inverter does is it tries to inject current into the grid at unity power factor in such a way that it zeros the real power measured at the CT. It is possible to zero your real power to within 1 half-cycle reasonably easily if you sample fast enough, but no inverter with this feature does (you could also do less than 1 half cycle with more effort). It does however mean that from the grid's perspective you use a lot of reactive power and no real power, which has other implications.
That does not explain this situation, and without knowing the control algorithm in the Sunsynk, it will remain a mystery.

These are just my best guesses.
Firstly, the inverter itself is not 100% efficient, and will cause some heat losses losses all by itself in the MPPT and the charger, also with the display and the wifi monitoring. That would explain why the solar input Watts exceed the sum of the load and the charging Watts.
Other than that, it is basically a grid-tied inverter, even if you're not exporting power, so it must synchronize its phase and frequency with the grid, and that invariably must mean taking a small amount of reference power from the grid.
The only other thing I can suggest is to ask the manufacturer, ie. not Sunsynk, but Ningbo Deye.

I think this is it. I say this because I don't own a Deye/Sunsynk and I see the same behaviour. 

If loadshedding is a factor, the suggestion would be to drain the battery to maybe 30-40% under the system work mode, then go back to grid, and leave a margin for further depletion to DoD of 20% set under the battery shutdown page, in case loadshedding hits in the early hours of the morning.

Unless you are a massive power user your batteries will most likely never go below 40%
you mention a base load of 450w/hr
 
Try reduce that base load to under 300w/h then you should never go below 40%, that's from my trials running off-grid.

Is there a way to run the A/C for shorter periods or an at "eco" setting?

Yes 20% SOC (80% DoD) should be fine - will give you a bit for those unforseen circumstances.How much does your AC use overnight for interests' sake?

Set in the system mode page. The answer to the first part is IT DEPENDS. I think the batteries can go to as low as 90% DoD with no issues BUT its up to you if you want to run them that low - most users leave a margin for unexpected outages / early morning LS before solar kicks in. The setting on the Battery shutdown is the absolute lowest your batteries will deplete to before they and the inverter shutdown

Approx 450W/hr from 5pm to 8am plus about 1.5kW for the cooker. That's 8.25kW so it's touch and go if the batteries make it through the night using the 20% figure

Thanks Mzezman, the quoted DOD on these batteries is 100% according to https://www.sunsynk.org/lithium-batteries. However another supplier site quotes "6,000 cycles with 80% DOD". 'So I assume setting 20% will give me a safety margin and be ok.