为什么这个椭圆体有点倾斜

Question

这个椭圆体为什么有点歪斜，上面还有黑色条纹呢？

import graph3;
import three;
size(0,200);
currentprojection=perspective(2,0,0.85);
real a=2;
real b=2;
real c=1.7;
triple f(pair z){
return (a*sin(z.x)*cos(z.y),b*sin(z.x)*sin(z.y),c
*cos(z.x));
}
surface s=surface(f,(0,-pi/2),(2*pi,pi/2),Spline);
draw(s,lightred);
draw(O--2.5X,L=Label("$x$",EndPoint,WSW));
draw(O--2.3Y,L=Label("$y$",EndPoint,ESE));
draw(O--2.2Z,L=Label("$z$",EndPoint,N));