A UK customer's $100 charge costs $5.70 in fees, not $3.20

Stripe's 2.9% + 30¢ is the domestic rate. A card issued outside the US adds 1.5%, and if you charge in the customer's currency and settle in dollars, conversion adds another 1%.

A $100 charge on a US card costs $3.20. The same $100 from a customer in London, charged in pounds, costs $2.90 + $0.30 + $1.50 + $1.00 = $5.70. Same sale, $2.50 more in fees.

Forty of those a month is $100 a month, $1,200 a year. Nothing in your sales report shows it. The fee is simply bigger on those charges, so the payout is smaller and the gap looks like rounding.

If a third of your customers are abroad, put that share into the fee calculator. The international and conversion fields are there for exactly this.