From my experience none of them are perfectly synchronised. I have two arrays and two gen. meters and neither read the same as the inverters; Just submit the gen meter reading.

Only the consumption and generation meters are properly calibrated, so those are the ones that are "right" by definition.
I usually apply a blanket correction factor, determined from the data. This is not entirely accurate as the difference gets relatively larger at lower power levels. The more accurate your data is, the more you can play around with trying to deconvolute all kinds of subtle effects. Not really worth it in my opinion..

Could losses in the inverter not be part of the story ?
In industry I always expect an inverter to use about 3% power (minimum)

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/454144/Electricity_Determination_Report_2003-2014.pdf

Selected quotes:

National Measurement& Regulation Office
Electricity Meters - Disputed Meter Accuracy Report June 2015

3.1 The limits of meter accuracy for meters approved under GB national legislation are prescribed in The Meters (Certification) Regulations (SI 1998/1566)4. Unlike most other countries, there is no additional tolerance for electricity meters operating in-service and they are required to maintain the same limits of accuracy as when new.

3.2 Electricity meters are considered “accurate” if the permitted margins of error do not exceed +2.5% to -3.5% throughout the entire load range at which the meter is designed to operate.

3.3 For MID meters the limits of accuracy at reference conditions are derived from the harmonised standard BS EN 50470-3:2006. The accuracy tests take into account the additional errors due to variation of influence conditions. However there are relatively few MID electricity meters installed in GB and, to date, only a handful have been submitted for determination.

MID Accuracy Class Maximum Permissible Error Limits

Electricity Meters (Class A)
±2.5% at minimum current flow;
±2.0% at one fifth of maximum current flow; and
±2.0% at the maximum current flow.

Electricity Meters (Class B)
±1.5% at minimum current flow;
±1.0% at one fifth of maximum current flow; and
±1.0% at the maximum current flow.

Electricity Meters (Class C)
±1.0% at both minimum current flow;
±0.5% at one fifth of maximum current flow; and
±0.5% at the maximum current flow.

end quotes.

So a meter approved to the EU/EN standard might be more accurate then the GB standard meter.

I have never really taken much notice of the difference between the PV generation according to the Enecsys monitoring system and the generation meter as I only use the former to check that all the microinverters are working. As I am paid only on what the meter indicates, then that is what I record.

Like Mikel's observation above, the weather is so gloomy here at the moment (west Wales), we have had three days in a row with zero PV output. This has never happened before in the 8 years we have had our PV system!