Yeah, on re-reading it's clear the shelf is shown as if it's for a taller person.  At least a few inches taller.  Same with the tool placement.  Her biomom?  An older relative?  A longtime staff member?  An assistant torturer?  If it doesn't predate Jelly it's built for more people.
Maybe Jelly really 
was possessed, after a ritual and blood writing. Which could have killed the original soul. Who might have been reincarnated by now. As a slime. Like the one who has extra buffs after taking over the body. 

 There's something weird going on...