Floss Drop Bobbin (Cross stitch)

I'll be honest, I have no real idea what makes this special or why the holes and such have to be the particular sizes they are!  But my daughter (who is into cross stitch) tells me they're awesome.  And she's had me make her over 50.   I've printed them in PETG to increase durability.  4 walls.  4 top and bottom layers. 10% gyroid.