The Preview step of the email template or campaign creation process is intended to preview design. At a general level the Preview page can verify that a merge tag works, but it is not intended to verify that the merge tag accurately represents records from a table. The Preview page does not contain logic that can evaluate tables with multiple rows.
For this reason, in the Preview step, the system will always pull the first custom table record.
The best way to verify that merges from a table are working properly is to use the Test step of a campaign. This is the only time that the system can accurately evaluate the segment and contact records to determine which row matches criteria.