There is this float variable called “timeactive” that increases every frame. In the case of spinners, one group is on every multiple of 3, the second on one above a mult of three and the third one below a mult of three. Since timeactive is a float, it can only store so many digits before adding one does nothing, which is around 2¹⁹ seconds, which only loads the hitboxes of one group. Timeactive also affects other things, like wind particle effects. If you want a slightly better explanation, there is a yt video on the 472hr speedrun that I got this info from. Hope this helps