This can happen with multi-person appointment services (semi-privates) -- when a staff member increases the service's 'Max customers', but doesn't toggle on 'Price is per customer'.

To check your Appointment Service's settings, click Appointments > Services > locate the service and edit it > After toggling on 'price is per customer,' check that the prices in dollars and credits are both correct for one person > then Save.

To preview your customer's scheduling experience, navigate to the Schedule, then click the 'Checkout' page link above the schedule.