Because each answer is assembled from the pages fetched for that asking, and the searches vary. The core of the list, the companies on most of the pages, is stable; the tail changes. That is why one asking proves nothing and fifteen questions across four engines, repeated monthly on the same questions, give a number you can act on.
What is stable
The companies that appear on most of the sources: the review site's top five, the names in every roundup. They are named in nearly every asking. A company in that core has high share of voice; a company in the tail is named one time in three.
What the report does about it
It asks each question once per engine and treats the answer as a sample; it reports share of voice across all answers, not any one; and it keeps the questions so the next month's check is on the same ones. Movement is measured against the same questions, not against a different asking.
Read next
How the AI visibility score works · Share of voice · Posture
Ask it about your own company
Your website, region and work email. The score appears in about three minutes, and the report is a link you keep.