As a consultant who focuses on providing advisory support to engineering teams developing ultrasonic transducers, I hear the following statement quite a bit:

"The new batch of transducers is different than the last one! We used the exact same materials, the exact same assembly steps, and we tested in the exact same way. But we have different results!"

I think ALL of those statements/claims/assumptions can be challenged despite their external truth.

If performance is different, there is a physical or system difference. Duh!

The problem is not that the same methods creates different results.

The problem is that a method to characterize the system/physical differences causing difference in performance has not been developed or implemented. Stop "kicking the can" and dig in to the meaty analytical and experimental work that will give you clarity instead of confusion.