Method for measuring tire tread depth?

Auston

Tires are required to be changed when the tread depth reaches 2/32".
At the dealership, I found on a brand new rear SP tire (Bridgestone
Potenza RE050A 245/40R19 94W) that the tread depth measurements are:

7/32" on the outside channel and
10/32" on the three inside channels

The tread depth is pretty different from the outside channel to the inner
three channels. It was my understanding that the tread depth was
supposed to be uniform across the tire when new- detecting uneven
tire wear is based on tread depth being uniform to begin with.

Also if you notice in pictures from AK370Z that the wear indicators/bars are only in
the inside three channels. So does one ignore the outside channel and just measure
the depth at the three inner channels containing the wear bars? Another method?
google and the370z.com search has been no help on this one.