Back on topic. With the transmission being a closed system, Mazda probably puts in the minimum amount they know is sufficient, and they probably do it very precisely and repeatably.
This is hypothetical, and I pretend no special insight into automatic transmissions or Mazda's in particular, but Imagine the safe operating amount is 80-100 units of ATF. They may say in the owners manual or indicator to fill to 90 because they are allowing margin for error.
At the factory they may choose to put in exactly 82 units with a process tolerance of plus or minus less than one unit. Then inspect or audit that and ensure that their process remains statistically in control.
The system may perform just as well, or even better, with 82 as it would with 90. But they can't trust customers to put exactly 82 when 79 might lead to premature failure.