A high proportion of content being excluded from match detection can indicate that a bibliography exclusion keyword may have been added only once to the paper, and possibly within a content table, which means that any text beyond that point will not be checked by the Turnitin service for Similarity.
It is important to ensure that the bibliography exclusion keyword used in a content table is also used as a header for the bibliography section of the paper, as Turnitin looks to exclude text being checked for Similarity from the last instance of the exclusion keyword occurring.
Keywords found as standalone words in a row may trigger the start of the match exclusion. A list of keywords can be found by clicking here.