Basically, the count in a segment is not always synced in real-time. It gets updated when you open or update the segment. However, the target audience for a campaign will be updated accurately, as it is calculated based on the segment conditions.
You may notice that the segment count appears different initially, but if you open or update the segment, the count will refresh. Also, MailBluster will use the updated recipient list when sending a campaign. While the displayed segment count might sometimes appear different, rest assured that the campaign will be sent to all recipients who match the segment conditions.