Why MATLAB function fan2para is not able to generate sinograms with odd number of views?

img = imread('example.jpg');
img = rgb2gray(img);
sino_fan = fanbeam(img, 538.52/0.710938, 'FanRotationIncrement', 360/1968, 'FanSensorGeometry', 'arc', ...
                       'FanSensorSpacing', (1.0239 * 912) / (946.746 * 2 * pi) * 360 / 912);
[sino_para, a, b] = fan2para(sino_fan, 538.52/0.710938, 'FanCoverage', 'cycle', 'FanRotationIncrement', 360/1968, ...
                               'FanSensorGeometry', 'arc', 'FanSensorSpacing', (1.0239 * 912) / (946.746 * 2 * pi) * 360 / 912, 'ParallelCoverage', 'cycle', ...
                               'ParallelRotationIncrement', 360/3, 'ParallelSensorSpacing', 1.0, 'Interpolation', 'pchip');

>> b

b =

     0   180

For the above code, I am trying to convert the fan-beam sinogram to a 3-view parallel-beam sinogram with ParallelRotationIncrement 360/3 = 120 degrees. But it turns out that the increment is 180 degrees. Same thing happens to all odd number of view.

MATLAB version: '9.5.0.944444 (R2018b)'

Thank you.