Strange behaviour in Matrix Visualisation

I have a list of Sample IDs with their Testing Status. When I try to summarise them using a Matrix, an extra line gets added over what I believe I should see. If I drop a couple of Cards on the canvas to show me the underlying stats then they agree with what I should see, but the Matrix adds a line I don't think should be there.

Are there any tips for things to look out for that could cause a Matrix to mis-interpret the summary?

Here is my example. 

Card 1 counts the Distinct number of Test Status values. It looks correct at 2.

Card 2 counts the number of Sample IDs. It looks correct at 184.

 However although the matrix is looking at the same data, it believes there are three categories of Test Status, two of which have the same name.

Any ideas on how that might happen? The total number of samples is correct so I don't think there is a duplication. The Count Distinct on Test Status is correct at 2 so Power BI seems to think there are only two distinct values. What could cause the matrix to think there are three?