How are the ERA5 single level datasets computed?

I have been comparing the ERA5 single level datasets and EAC4 multilevel data and I am confused.

If I use the EAC4 data to compute the total column concentrations for ozone, for example, I get a different value than I receive from the ERA5 single level dataset.

To be more precise, I use the mass ratio data from EAC4 and the surface pressure value for my location. Then I compute the pressure profile using the PSAB method with the 60 level specification. Using this pressure profile I compute the total column concentration of ozone.

I do notice, that my way of computing the total column concentration becomes consistent with the single level data from ERA5, if I assume 1013 hPa surface pressure and ignore the actual value.

So, finally, to come back to my question:
What is the relationship between the ERA5 single level data (total column ozone) and EAC4 multi level data? Which of these two values is computed from the other? Am I right in suspecting, that the default L60 level definitions, with 1013 hPa surface pressure, are used to compute the ERA5 single level value from the EAC4 multi level data?

Ah, I just had another thought: Is it possible, that the single level data is computed using the Mean sea level pressure, instead of the surface pressure?

the two datasets are produced independently. Here a table with some info:

Exp ID exp=eac4, class=mc Period covered 1940 - present
Period covered 2003 - present (though only 2003 - Dec 2022) Production Period Jan 2016 – end 2017, then continued in near real-time
Production Period Jan 2017 – Dec 2018, then continued in near real-time Model version IFS Cycle 41r2
Assimilation system IFS Cycle 42r1 4D-Var Assimilation system IFS Cycle 41r2 4D-Var
Spatial resolution 80 km globally (T255), 60 levels to 0.1 hPa Spatial resolution 31 km globally, 62km for the Ensemble of Data Assimilations (EDA), 137 levels to 0.01 hPa
Output frequency (temporal resolution) 3-hourly analysis fields,3-hourly forecast fields from 0 UTC up to 48 hours,1-hourly surface forecast fields from 0 UTZ up to 48 hours Output frequency (temporal resolution) Hourly analysis fields, 3-hourly for the Ensemble of Data Assimilations (EDA), Hourly forecast fields, 3-hourly for the Ensemble of Data Assimilations (EDA), up to 18 hours, with reduced frequency up to 10 days (not in initial release)
Anthropogenic missions MACCity (trend: ACCMIP + RCP8.5) & CO emission upgrade Stein et al. (2014) Uncertainty estimates From a 10-member Ensemble of Data Assimilations (EDA) at 63 km resolution
Biomass burning emissions GFAS v 1.2 Model input Appropriate for climate (e.g. CMIP5 greenhouse gases, volcanic eruptions, SST and sea-ice cover)
Biogenic emissions Monthly mean VOC emissions calculated by the MEGAN model using MERRA reanalysed meteorology (Sindelarova et al., 2014) for the whole period. Input observations In addition, various newly reprocessed datasets and recent instruments that could not be ingested in ERA-Interim
Chemistry modules IFS CB05 (Flemming et al. 2015, with updates as given in CAMS: Reanalysis data documentation) & Cariolle ozone parametrisation in stratosphere Variational bias scheme Also ozone, aircraft and surface pressure data
Aerosol modules See CAMS: Reanalysis data documentation Satellite data RTTOV-11, all-sky for various components
Input meteorological observations As in ERA5 (2003-2016?), OPS for later years New parameters ERA5 contains over 240 parameters on surface and single level alone, plus parameters on other level types. For specific parameters please compare the technical documentation linked below.
Input atmospheric composition observations See CAMS reanalysis data documentation Parameters removed A few parameters present in ERA-Interim are not available in ERA5. For availability of specific parameters please see the technical documentation linked below.
Aerosol used in radiation scheme Interactive active aerosols, i.e. aerosol fields from eac4 used in radiation scheme Handling of accumulated parameters Accumulated from previous post-processing
Ozone used in radiation scheme Interactive ozone, i.e. ozone field from eac4 used in radiation scheme Product main page Climate reanalysis | Copernicus
Parameters Full chemistry output archived as type=an every 3 hours (see parameter lists on CAMS: Reanalysis data documentation) Technical documentation ERA5 data documentation
Stratospheric chemistry No, but Cariolle ozone parametrisation in stratosphere and stratospheric O3 available. Additional innovations Long-term evolution of CO2 in RTTOV, cell-pressure correction SSU, improved bias correction for radiosondes, EDA perturbations for sea-ice cover
Product main page CAMS: Reanalysis data documentation
Technical documentation CAMS: Reanalysis data documentation