Spreadsheets and databases change over time. When you first set up a metric value import, you may have a spreadsheet that includes 10 metrics with values. The next month, there may be 12 metrics with values in the spreadsheet.
There's a good chance you also want to import values for these two new metrics, but they haven't been mapped to metrics in Scoreboard/QuickScore yet. For this reason, Connect now has an Unmapped Source Data report showing all source data metrics that haven't been mapped to any metrics in Scoreboard/QuickScore.