issues about file "FourWheelSteering.m"

[iDonghq]

Thanks to your wonderful work for these model, but I am a bit confused about "function [wheelSpeeds,steerAngles] = inverseKinematicsZeroSideslip(obj,vx,w)", in this function "phi = atan(w*(obj.frontWheelDist+obj.rearWheelDist)/vx)", I found that "phi = atan(w*(obj.frontWheelDist+obj.rearWheelDist)/(2*vx))" by using

Could you help me explain it?
thank you very much.