Why do some conferences have a variable amount of reviewers for each submitted paper? Why do some conferences have a variable number of reviewers for each submitted paper?

For example, I read on https://redd.it/9nomv1 that the AAAI 2019 conference has between 3 and 5 reviewers for each paper submission. Why isn't the number of reviewers the same for all submitted papers? Is that to make up for non-experienced or outside-the-area reviewers, or for reviewers who do not turn in reviews?
Editors usually invite more than the minimum amount of reviewers when they assign a paper for peer review, because not all invited reviewers will agree to do the review. A suggestion that I have seen in a guideline for editors (and which indeed mostly works well in my experience for this specific publication venue) is to invite 5 or 6 reviewers initially when aiming to get 2 reviews.

Occasionally an editor will be lucky and (almost) all invited reviewers will submit the review, leading to a higher than required number of reviews for this manuscript.

In other cases there will be less than the minimum number of reviews even though more reviewers have been invited, and the editor will have to invite even more reviewers.