Airfares are living things. A few common reasons:
Fares change fast. Airlines update pricing continuously; a fare cached minutes ago may already be gone.
Fare type. The lowest price is often a basic fare — personal item only, non-refundable, non-exchangeable. Baggage and flexibility cost extra, and each result shows this before you click through.
Currency and payment fees. Some vendors charge in a different currency or add payment-method fees at checkout.
We always recommend reviewing the full fare conditions on the vendor's page before paying.