In theory, Total Reach should be equal to the sum of Organic, Viral, and Paid Reach. However, due to the way that Facebook calculates these figures, this is often not the case.
The discrepancy is related to how the figures are estimated, which causes there to be differences in their corresponding estimation errors.
Here's a brief explanation from Facebook:
Gary Fitzpatrick · · Facebook Team
"The Insights team have looked into this and they have informed me that what you are seeing specifically is 'By Design'. This is because uniques have an estimation error. The estimation error can cause the total to have more error and be greater than the organic + viral. As long as the discrepancy is not too much, it is expected. Gary"