Most CAMS and C3S data data is produced and archived not on a Cartesian grid (a two-dimensional rectangular grid), but on a reduced Gaussian grid - think of it as a globe with a series of evenly spaced data points along each parallel, and parallels spaced at regular intervals. So near the poles you have only few points along a parallel, but close to the equator you have about many data points along a parallel. For example for ERA-Interim non-wave data this interval is about 0.75 degrees[1], and for ERA5 the interval is about 0.3 degrees.
When you download CAMS and C3S datadownload CAMS data, C3S data and other data from ECMWF, you can specify to get the output data in a Cartesian lat/long grid and at a custom resolution.
For ERA-Interim you can specify a custom grid on the data server . In this case the default horizontal resolution is web interface, using the ECMWF WebAPI or using the MARS client (if you have access to it). On the web interface the default grid for ERA-Interim is lat/long, with a default resolution of 0.75x0.75 degrees (about 80km), approximating the irregular grid spacing on the native Gaussian grid.
...