My guess is that Stripe has a service internally which predicts "how much of each currency am I going to have, and how much of each currency do I need to pay ou

My guess is that Stripe has a service internally which predicts "how much of each currency am I going to have, and how much of each currency do I need to pay out at the end of the day?" Then, they balance whatever currencies they can internally, and for the rest, convert to USD, before paying out to customers. The reason here is because Stripe wants to mitigate exposure to non-USD price fluctuation, e.g. if EUR dives against the dollar. Many companies do (or should do) some version of this rebalancing, e.g. FB cited on an earnings call last year that they lost a large amount due to forex fluctuations.