When you get a quote, that exchange rate is locked for 30 minutes. If you confirm the transfer within that window, you get exactly that rate regardless of how the market moves in the meantime.
If the quote expires before you confirm, we simply refresh it to the current rate — you won't be charged the old, expired rate by mistake.
This protects you from market volatility during checkout; it does not mean the rate is fixed for future transfers, which are quoted fresh each time.