The spacing between x-coordinates and y-coordinates is not equal. In most cases, this is encountered when the spacing between latitudes is not equal. The difference between latitudes decreases as you go toward the poles.
When you 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 not equally spaced, first create a netCDF feature layer. Then interpolate to raster.