There's really no good answer for that other than it's because of the shape of the lens/mirror. If you carefully work out the path of various rays, you'll find they don't converge to a single point. They come relatively close, so to a good approximation, you can consider the rays to be intersecting at a single point. When this approximation isn't good — in other words, when you can tell they don't meet at a single point — the effect you see is spherical aberration.
For a spherical mirror, for instance, rays from infinity intersect the axis as a distance [itex]l=\frac{r}{2}\cos\theta[/itex] from the center of the sphere, where r is the radius of the sphere and θ is the angle of incidence to the mirror surface. Then θ is small, you can approximate cos θ to be equal to 1. This implies then that the rays focus at a distance r/2 in front of the mirror. The rays for which the approximation cos θ≈1 isn't good result in spherical aberration.