Explanation
There are overlapping contacts in each of the channel is why the total contact count would be less than the total number of contacts from all data sources. Contacts are unique individuals who can be reached through one or more channels in Marketing Cloud, such as email, mobile, or social. Contacts are counted based on their Contact Key, which is a unique identifier for each contact. If a contact has multiple channels associated with them, such as email and mobile, they are only counted once in the total contact count. Therefore, if there are overlapping contacts in each of the channel, the total contact count would be less than the sum of contacts from all data sources