Same question, different engines: what stays fixed?

Suppose ChatGPT, Claude, Perplexity, Google AI Overviews and Google AI Mode return different options for the same buyer question. Before explaining the difference, I would record the exact question, dates, locale and account context.

This is a hypothetical comparison, not a fresh measurement. What else must be held fixed, and what cannot realistically be fixed?

If you have a public example you have permission to share, bring the answer and its context. I would like us to separate what we observed from the explanation we are tempted to give it.