Question

why is the calorimeter constant different for every bomb calorimeter ?

Answer 1

Calorimeter constant depends on the design of the calorimeter. Some may use 2.000 L, another might only use 1.000 L.

Even if you have two devices made by the same manufacturer, they ideally should be the same, but differences in the manufacture, age, dirt... could have small impacts on the value of calorimeter constant. When they build it, they tell the user to do the calibration, so there isn't any systematic error due to differences in usage.

Perhaps you made a calorimeter out of a styrofoam coffee cup. Maybe I made a calorimeter out of a large metal thermos bottle. There is no way that both of those devices would have the same calorimeter constant. It isn't a constant value like Avogadro's number or pi, but it depends on how each individual device is built.

In simple words I would say different bomb calorimeter have different calorimeter constant on the basis of their designing structure.

I hope you understand what I mean to say.