I think this is a result of the way the zirconia is manufactured. This is just a theory but it could have something to do with it.
There are three ways to make zirconia discs:
- Isostatic pressing (pressure from all angles, in a bag, underwater)
- Unilateral pressing (in a former or mold being pressed from the top and bottom)
- Chemical solidification (just uses a chemical and binder the hold the zirconia together, Glidewell famously uses this technique)
Unilateral pressing is by far the cheapest way to make zirconia discs, I would guess thats how the zirconia that you are using is made. Isostatic is much harder to do and more expensive of a process but it creates a uniformed zirconia disc in the process. Most high quality, more expensive, zirconias use Isostatic pressing.
I could be wrong but I think its worth adding to this conversation.