Thanks for posting your question and for your interest on the Foreign Exchange Rates API.
The Foreign Exchange Rates API behavior you are seeing in sandbox is expected since this API uses simulated data in the sandbox. When the Foreign Exchange Rates API moves from sandbox to production the exchange rate that you will see will be correct.
Please let me know if this information was helpful and if there are any other questions I can help you with. I am glad to be of assistance.
Visa Developer Team
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