The spacing between x-coordinates and/or y-coordinates is not equal. In most cases, you encounter this problem when the spacing between latitudes is not equal. The differences between latitudes decreases as you go toward the poles.
To create a netCDF raster layer from a netCDF variable, the spacing between x-coordinates must be equal and the spacing between y-coordinates must be equal. If the coordinates are unequally spaced, create a netCDF feature layer, then interpolate to raster.