It's difficult to say why you're getting the wrong information without seeing the workbook. Would you be able to post it or a sample as a twbx file?
Tableau was very responsive. They were able to help me. Here was their response from the case I created:
"It appears that you have dimensions placed on the Filters shelf, but not in the view itself. The Filters shelf only filters the primary data source. Therefore, if the filters should apply to the secondary as well, the linking fields will need to be added to the view. In Tableau 7, linking fields must be present in the view, otherwise they will not be taken into account when pulling data from the secondary data source. Currently, the workbook is pulling all data from the secondary source associated with each zip code, regardless of the number of bedrooms or rented year.
To resolve the issue, follow these steps:
1. Click on the primary data source in the Data pane
2. Drag [# Bedrooms] from the Dimensions pane to the Level of Detail shelf
3. Drag [Listed Date] from the Dimensions pane to the Level of Detail shelf"
It is worth noting that Tableau 8 will no longer require linking fields to be added to the view. Tableau 8 is currently scheduled to be released later this month, but the release date is always subject to change."