To properly test repeater groups, you should set up a campaign with the appropriate segment and use the Test function in the campaign process.
Using the Preview step on a template or in a campaign will not provide accurate results. The preview page has no logic applied that can tell the repeater group which records to display. Because of this, the Preview step will always display all records from that table.
The Test function will apply the segment logic to limit the repeater group to only records that meet the segment logic for the test email received.