Searching for Current Member = No returns contacts who are current members if they also have a non-current membership
If you search in Advanced Search or Find Memberships with Current Member = No, you will get all contacts who have a membership which is not current. This is not the same as saying they aren't a current member, because the contact could have another membership which is current. This is probably quite common as many contacts will buy a new membership instead of renewing their past membership.
In the case that a contact has, for example, one expired membership and one current membership, they will be found when searching for Current Member = No and also for Current Member = Yes. Confusing!