How to determine Supernetting/CIDR IP address range?

Just when I thought I had an understanding on how to calculate the IP address ranges via Supernetting/CIDR I am stumped as to how did MS get 192.168.31.254 as the end of the range.specifically where did the ( .31 ) come from?
For example, to express the range of addresses for the address prefix 192.168.16.0/20:
The first IPv4 unicast address in the range is 11000000 10101000 0001000000000001 (host bits are bold), or 192.168.16.1.

The last IPv4 unicast address in the range is 11000000 10101000 0001111111111110 (host bits are bold), or 192.168.31.254.