Exchange rates move constantly, driven by interest-rate differentials, trade flows, capital flows, and expectations. The 'mid-market' rate is the true midpoint between buy and sell prices; banks add a spread on top.
See also
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Open Decision Lab →Frequently asked questions
Fixed vs floating exchange rate?
Floating rates move freely with market forces. Fixed (pegged) rates are held to a target by the central bank, sometimes with capital controls.