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.
