This basically comes from there being unanswered questions after the show’s 3 season run, and so that we could see more of certain characters after the season 3 finale did not look like it would be the last we ever see them unless able to return in another series: https://www.change.org/p/mako-island-of-secrets-is-a-good-show-but-it-s-finale-had-to-many-questions-so-they-need-to-answe-i-want-mako-island-of-secrets-to-continue-for-new-seasons

Having 3 seasons total seems to have been intended by the creators, but why season 3 would have 16 episodes as opposed to 26 is a mystery to this day.