When downloading ERA-Interim data the default horizontal resolution is 0.75x0.75 degrees. If you specify a higher resolution the data will be interpolated and appear smoother. However, the underlying data is still the same, so the improvement is only a visual effect.
More detail:
The ERA-Interim 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 0.7 degrees. So near the poles you have only few points along a parallel, but close to the equator you have about 500 data points along a parallel.
When you download ERA-interim data, the grid points are projected into a Cartesian lat/long grid. The default horizontal resolution of the output is 0.75x0.75 degrees, approximating the irregular grid spacing on the Gaussian grid.
If you specify a different output resolution, the system does not use more input points, it merely interpolates the same data to a higher resolution. This makes the output look smoother, but does not increase the accuracy or the precision of the data.
