The tread depths were taken at the end of last season (March '21) after an alignment at the dealer (November '20). The summer set of tires are all wearing at the same rate as were the winter tires until that last reading.
The 9/32nds tread is from a replacement tire due to a road hazard (November '20); the 6&7/32nds treads are from three years of winter driving; and the 4/32nds tread from the right rear after the last rotation is the confusing one.
Rotations done between 6,000 and 7,000 miles (usually).
As for treads of 2/32nds: Both the local tire shop and the tire manufacturer’s customer support line stated this value as the legal limit but added that I should replace the tires sooner if I felt unsafe on the road. Neither party indicated a different standard for snow tires based on “needed” traction/control.