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Usually they use that "uncertainty" to trick you in accepting that (usually) higher conversion rate on the spot. People seem to love the fact that "that's it, that's the amount".
Some banks/apps/sites usually give you that as a default option and that alone, for me, it's a sign that's a trap.
I think once I noted down the amount offered and then verified later what was actually taken and there was a noticeable difference (I saved), but, as usual, YMMV
Same re not accepting conversion at the time of purchase.