The retrieval of land-sea mask from ERA5 or ERA-Interim on its archived grid (grid=av), or on a reduced Gaussian grid and on a regular lat/lon grid (e.g. grid=2.0/2.0), gives different results. The data on its archived grid or on a reduced Gaussian grid, has fractional values in the range 0 (sea) to 1 (land), instead data on a regular lat/lon grid is binary, with values 0 and 1.
This is due to the ECMWF's interpolation tool MIR which by design interpolates a land sea-mask to a reduced Gaussian grid as fractional values, including for regular lat/lon grids.
Notes