I would guess it's because the machines embroidery foot needs the room. The foot can sew closer on one end inside the hoop than the other end. So you would center your embroidery design center with the center mark of the hoop template. The hoop is actually larger than the machines embroidery area.
Can't figure out how to post a pic, so this link shows what I mean about the hoop template. The blank section at the top is space for the foot. The machine can only sew inside the red (design) area:
https://www.sewingpartsonline.com/5-...SABEgLG6PD_BwE