Why do size charts sometimes create more questions than they answer?

A size chart is only useful when it explains what the numbers describe. Sometimes they are body measurements; sometimes they seem to be the garment laid flat. Stretch, cut and the intended fit can change the choice again. Then a customer review says it fits perfectly without mentioning anything comparable. What information makes ordering clothes online less of a guessing game? Interested in clear measurement instructions and useful product photos, rather than comparing anyone's body or assuming one sizing system works for everyone.