With a variable search radius, the number of points used in calculating the values of the interpolated cell is specified, which makes the radius distance vary for each interpolated cell, depending on how far it has to search around each interpolated cell, depending on how far it has to search around each interpolated cell to reach the specified number of input points. Thus, some neighbourhoods can be small and others can be large, depending on the density of the measure points near the interpolated cell. You can also specify a maximum distance, in map units that the search radius cannot exceed.