Why is their 2 different minor version numbers for the same version and SP level of SQL?

I've noticed SQL has 2 different minor version numbers. I'm trying to understand why this is, what it means, etc.

The standard product version format for SQL Server is MM.nn.bbbb.rr. Each segment is defined as: MM - Major version, nn - Minor version, bbbb - Build number, rr - Build revision number.

At  https://  sqlserverbuilds  .blogspot  .com/ , you will see a table containing a list of SQL Server Versions.

You'll notice that SQL 2016 SP1 has version 13.0.4001.0 and 13.1.4001.0, for example. represent SQL 2016 SP1.  The pattern of having 2 minor versions per service pack level goes back to SQL 2008.

1. What does the minor version .0 and .1 mean?
2. Where can I learn more about the differences?
   
3. Why there are 2 minor version numbers for the same SP level?

Thanks in advance!

Terry Musser