Differences between data access methods and file formats

We're looking at ERA5-LAND 2m-temperature, but can't explain differences between GRIB and NetCDF files downloaded at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-land?tab=form with another NetCDF file acquired from the CDS API (via Python client).

Here are 3 time-series in °C from the 3 separate files for 1990-01-01 UTC times at location X=-122.0728 Y=39.75376:

                    date   t2m_api    t2m_nc  t2m_grib
 1: X1990.01.01.00.00.00 13.221094 13.237744 13.237451
 2: X1990.01.01.01.00.00 11.665186 11.681625 11.681787
 3: X1990.01.01.02.00.00 10.072168 10.095596 10.095850
 4: X1990.01.01.03.00.00  9.000879  8.965128  8.964990
 5: X1990.01.01.04.00.00  9.196192  9.182172  9.182031
 6: X1990.01.01.05.00.00  8.310938  8.317065  8.317285
 7: X1990.01.01.06.00.00  7.488183  7.531720  7.531885
 8: X1990.01.01.07.00.00  5.521875  5.556852  5.556543
 9: X1990.01.01.08.00.00  5.076074  5.118164  5.118311
10: X1990.01.01.09.00.00  5.546777  5.577560  5.577295

Here are plots of the 3 files at 1990-01-01 00:00 UTC:

API method (in °C):


GUI NetCDF format (in K):


GUI GRIB format (in K):


We use R raster::extract() to overlay the point location to the rasters (but we have the same results in QGIS).

Is there any reason why we see small differences between the separate files? Max difference is in the order of 0.8°C (between API and GUI) over the month of January, but this is a little concerning for reproducibility's sake.