anyone know how the Preserved Collarbone spawns?

This item can spawn from the Abyssal Cache at a certain probability,
yet this type of chest is extremely rare, while all other chests are quite common. I suspect specific conditions are required for its spawn;
it may be related to the three monster factions, or it might just generate randomly.
Does anyone here have any relevant information about this?
Is the spawn of all boxes from the Abyssal truly random?

I have also noticed how much rarer this is compared to the one for armour and the other for weapons. Are the drop rates just different for a specific reason or are we missing something?