When the text content of a segment produces a 100% match against the TM but the source segments in the document and in the TM contain different formatting, a penalty is applied to the match. In the sample file, Segments 1 and 2 are the same, except that Segment 2 contains some text in bold. When you reach Segment 2, the Translation Results window shows the following match:

The yellow arrow symbol under the match percentage tells you that a Different formatting penalty has been applied. This alerts you to the need to edit the match to ensure that the formatting is consistent in your source and target segments. SDL Trados Studio also has a Missing format penalty that would have been triggered if the bold formatting were in Segment 1 and not in Segment 2.