Why this happens
The model predicts a plausible continuation of the text, rather than verifying facts. If data is lacking, a plausible fabrication comes out just as smooth as the truth.
What a website owner should do about it
Do not expect the assistant to figure it out on its own. The clearer the prices, deadlines, and conditions are stated on the website, the fewer reasons there are to invent them – and the more often the answer matches reality.