System merging assets

We want to know what causes the system to merge a debit line and a related price variance credit.  When we look in Prepare Mass Additions for a given PO, we're seeing that separate labor charge lines related to the installation of a number of displays have been merged by the system to related price variance credit lines.  Each labor charge was $4,500 on the PO but the vendor invoiced only 3,500 for each installation.  If we could merge each of the merged, debit + variance credit, lines to the related display, we would be fine. However, the system won't