IPv4 Default Gateway address

it has always bee 1 digit less than the assigned dynamic ip address given to me by my ISP. But now it is 1 digit higher

 

 

These are just made up IP address for illustrative purposes

example: Say my assigned IP address given to my ISP is 51.46.211.100.  In the past the IPv4Default gateway  would be 1 digit less 51.46.211.99

 

But now if my assigned IP address is  51.46.211.100, the IPv4 Default gateway is 51.46.211.101