The Balance page always shows a block for your store currency, and adds another block for every other currency that Stripe reports a balance, pending payout, or reserve in. That happens if you have ever accepted a payment or received a transfer in a second currency, or if your Stripe account's default (payout) currency differs from your store currency. Each block has its own KPI cards and a Balance Summary · <currency code> table (for example Balance Summary · USD, Balance Summary · CAD).

Your store's base currency is set on Business Settings > Configuration, in the Business: Payment tab — in the current version that field has no friendlier on-screen label, so it shows up literally as currencyCode.