This is the response I got from wrfhelp two years ago.
"This is because WRF/WPS involves lambert corformal, polar stereographic, mercator and lat-lon projections. The transformation for the first three projection is isotropic. The isotropic transformation requires dx/dy = constant everywhere on the grid."
Regarding the math of it or the explanation of the GIS part of it
you can ask about the details here - http://gis.stackexchange.com/
There are GIS and mathematical experts who can guide you on that answer.