The most common reason is an account billing configuration for what to do "when customers are unsuspended."
The account level setting can be changed in the Admin UI under Settings >Financial >Billing Preferences.
- Charge for all missed periods will produce a single invoice with line items for all missed charges.
- Charge for current period (Prorate if applicable) will charge for the current period from the day they are unsuspended until the end of the current period.
- Charge for current period (Do not prorate) will charge for the current period from the start day of the current period for the full period amount.
- Do not charge for any missed periods will skip charges until the next billing period arrives.