Best as I can figure it, it’s how far away from the perfect focus it is.
Inside a parabola, there’s only a single point in 3D space which is the focus. And an LED chip isn’t a point-source, but a flat square, a tile. So the farther away you get from that (at the sides of the chip, especially at the corners), the more out-of-focus it is. So the hotspot gets distorted.
That’s why especially in a G3 (flip-chip), which has horrible hideous tint-shift as you go off-axis, the worse the corona, which is light that hits the reflector not from the focus but slightly “off”, and doesn’t get focused parallel but again slightly off-axis, and is tinted that horrible urine-yellow color.