Why a list's number changed on its own

Counts move without anyone editing anything. Which number moved tells you why.

The mailable number dropped, the total did not

Nothing was removed — people became unmailable. Someone unsubscribed, an address bounced, or a complaint came in, and each of those suppresses automatically.

This is the system working. See What "N mailable from M" means.

A segment's Members changed

Segments re-evaluate themselves, so their count follows behaviour. A "bought in the last 90 days" segment shrinks as older purchases age out, with nobody touching it.

Check Last evaluated to see when the figure was worked out — it is not live. See When do segments update?.

A list's total grew

Something is feeding it: a store sync, an ad connection with routing rules, the API, or an import someone else ran. Filter Contacts by Source to see which. See Where contacts come from.

The total dropped

Lists do not lose members on their own — this one is a person or an integration. Someone deleted contacts, or removed them from the list.

Deleting a contact removes them everywhere; removing from a list leaves them in your account. Both are deliberate acts, so this is the case worth asking about rather than explaining away.

A campaign reported fewer than the list holds

Expected, and the same four filters as always: suppressed, invalid, no consent, de-duplicated across overlapping audiences. See Why fewer people received the campaign.

Next: work out which number moved — total or mailable — before investigating. They have completely different explanations.