As a joke. Back when 2 came out, they promised there would never be a version 3. I.e., no breaking changes. Then they realized they did need to make a breaking change, so the only way to be true to their promise was to skip version 3.

Exponential versioning. Next major would be 8.

Carson promised they'd never do a version 3...but he never said they couldn't do a version 4.