When building out a Content Tag, you have the ability to select the 'Preview Matches' button to view the posts that meet the specific content tag criteria that you have built out.
When selecting 'Preview Matches,' the posts that show are based on all of the available posts that meet the criteria of the Content Tag. This includes every Account that you have identified in the Content Tag and every available post, regardless of the date.
However, when you add the Content Tag into a Campaign, you are looking at a specific date range. Therefore, all of the posts that show when you preview the Content Tag will not be in the campaign. Furthermore, within a campaign you may restrict the Campaign to various accounts.
This means the total number of posts within a Campaign will be less than when you previewed the total number of posts matching the specific content tag based on the constraints created within the Campaign.
If you have any further questions regarding the Posts when Previewing the Posts in a Content Tag and the number of posts you are seeing within a specific campaign feel free to reach out to your Account Manager, or contact firstname.lastname@example.org.