This usually happens when a static target group (not an auto target group) was used for mailing.

Addresses that were unsubscribed before the mailing started, but are included in the (static) target group selected for the mailing, are shown as delivered in the campaign statistics. To avoid this, the target group must be cleaned of unsubscribers. For auto target groups, this happens automatically once a day.

An unsubscribed address (unsubscribe=1) is automatically filtered out in the delivery process, i.e. not contacted - but it can be included in the statistics if static target groups are used.

Translated with www.DeepL.com/Translator (free version)