At the moment, segments update daily even for the scheduled campaigns. If you schedule a campaign to be sent out at a later date, the segment will look different than the one at the moment of campaign creation. For example, you schedule a campaign to be sent out in a few days and make a segment of the latest subscribers (added in the last day). The system calculates that there are a certain number of contacts that the campaign will be distributed to. However, at the moment when the campaign is being actually sent out, there will be no email recipients because the system updates the segment and realizes that there are no new subscribers in the last day.
The segment updating should not be applicable to scheduled campaigns.