Facebook defines total presence impressions, highlighted in the black box, as any content from your Page or about your Page; this includes posts, stories, ads as well as other content or information on your Page. Impressions by source, highlighted in the blue box, only includes impressions from posts and stories. Since impressions by source does not include all content types, total presence impressions are often higher than impressions by source.

You can see Facebook’s exact definition of each metric in the API reference documentation.