I don't know anything about the 1 -> 6 jump, but the jump to 18 was because they switched to time-based versioning last year (2018). First release of 2019 was 19.0.
the 1 -> 6 jump was because they decided to drop the first number.
Yes, they had reasons, but changing version schemas twice in a couple years is a great way to confuse people in the interest of just doing whatever the author likes.
This is what I feel like Pip did. https://pip.pypa.io/en/stable/news/
it went from 1.5.6 to 6.0, and later from 10.0.1 to 18.0.
Ok.