Why antenna factor AF is not maximum at 90 deg when spacing is different from lambda?

Hello! I am learning antenna factor for 2d microstrip array (http://www.antenna-theory.com/arrays...oDuniform.html). I am using weighs = 1 for all patches. When i am using wavelength spacing dx=lambda, dy=lambda, then AF graph looks okay (maximum at 90 deg). But any spacing different that lambda gives sidebeams. I can provide images later.