Why the exchange rate gets frozen

Exchange rates move daily. If ContoTrip converted every expense at today's rate, the total for your trip to Thailand would change every morning, months after you got back.

The rate belongs to the expense

So every expense freezes its rate: the one from the moment of capture. If you add an older receipt later, the app uses the historical rate of the receipt date – not today's.

What if the bank calculates differently?

It almost always does. Your bank uses its own rate and often adds a fee. That is why the rate is editable on every expense: just enter the value from your card statement, and the total matches what was actually charged.