Why is the revenue on Customers lower than on Home or Revenue?

The figure on Customers adds up what each customer on the list has paid you, so it always matches the list underneath it. Money you recorded without choosing a customer — a cash sale entered by hand, a deposit nobody was attached to — is real revenue, and Home, Money and Revenue count every cent of it, but it belongs to no customer, so it cannot be in a per-customer total. When there is any, Customers says so under the figure, for example "$1 recorded without a customer is not in this total". To put one on a customer, open the payment on Money and choose the customer under Who paid; it moves into that customer's total at once.