Difference in multilooking calculated manually and calculated by the software?

Hi all,

I have an ALOS PALSAR image with the following parameters
Incidence angle = 38.67 deg
Range pixel spacing = 9.36
Azimuth pixel spacing = 3.20
Upon calculating the number of looks manually, I should get 4 number of looks in azimuth and 1 in range to generate square pixels of 15 m.
However upon selecting GR Square Pixel option, SNAP allows to change only the number of looks in range which is 1, but azimuth looks is fixed with 5 number of looks generating square pixels of 16 m.

What internal parameters does the software add to get such a difference in the result?

Thank you!