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

1. For NetCDF output on ECMWF's Web API, or if you intend to convert to NetCDF with ecCodes, you have to retrieve data on a regular lat/lon grid.
2. The behaviour described above applies to data retrievals through ECMWF's  Web API or MARS client. Data retrievals from the Climate Data Store (CDS) use ECMWF's interpolation tool MIR, which by default interpolates the land-sea mask to fractional values, including for regular lat/lon grids.