Where the difference between answers comes from
A language model creates text step by step, choosing each next word from several likely options. In most chats the choice is partly random, so the same prompt leads to different wording and sometimes to different decisions. This is not an error but part of how such models work: thanks to randomness they do not give the same thing every time and can suggest different ideas. For creative tasks it is a plus; for tasks that need accuracy and repeatability it is a minus that must be managed.
What in a prompt makes answers more stable
The less freedom you leave, the smaller the spread. Describe the output format: a list of five items, a table with set columns, a letter template with required parts. Set criteria: length, tone, what not to do. State who it is for and why. A vague prompt like “prepare a report” gives a wide spread, while a prompt with an exact structure and conditions narrows it. Add checkable requirements and ask the AI to check its result against them before replying.
Examples beat explanations
The strongest way to stabilise a result is to show one or two samples of the desired answer. The AI repeats the structure, length and style of examples far more precisely than described in words. A sample should be exactly how you want the result, with a note about what matters in it. If there are two examples, let them differ in content but match in form: then the model understands what to repeat and what to change. This often gives more than long instructions.
For important decisions, compare several versions
If something important depends on the answer, do not rely on a single attempt. Run the prompt two or three times in new chats and compare. Parts that match are probably more reliable; diverging parts need checking against sources. You can also ask the AI itself to propose several options and rate them against given criteria: this does not replace checking, but makes the spread visible and manageable. For regular automated tasks programmers use generation settings that reduce randomness, but in ordinary chats they are usually unavailable.
When differences are fine
For ideas, headlines, wording options and creative tasks, variety is useful: you can ask for five different versions and pick the best. Worry when facts, numbers and conclusions change. That is a signal that the AI is not relying on reliable material, and you must either give it a source or check the result yourself. So first decide what you need: one exact result or a set of options, and set up the prompt accordingly.