Proration occurs when you receive services for only part of the billing cycle. There are several scenarios when this can happen, notably when upgrading or downgrading a subscription during the billing cycle.
When you change your plan during a billing cycle, a proration line item is applied on your next invoice. If your subscription is billed on the first of the month and you changed it to a more expensive plan, you will receive a credit for the unused portion of the old plan. Then you will be billed only for the remainder of the month for the new plan.
The prorated amount is calculated down to the second by Stripe. We calculate the difference between the two plans based on the time the request was made to change your subscription, using the current billing period’s start and end times.
In other words, the subscription is prepaid, and given that you would have already paid for this cycle, the system wouldn't be able to backtrack and charge you for any adjustments you make in between cycles. Instead, all adjustments will be applied to the next invoice where it will be added to that new cycle.