GCP in Auto Mode creates subnets in many regions using a predefined list of subnets. But there is no documentation or reference that I could find on why these ranges are selected?

Any know of the story behind the subnet ranges?

The auto mode essentially creates in 10.128.0.0/9 CIDR with a /20 mask.

Reason/History --> I guess more of practical. One need to have big enough and scalable for future regions. So the /20 & Class-A looks apt.