This discrepancy may be due to sessions that are counted in multiple channels. For example, during a single session, a user might arrive via organic search, then return direct within a short time period. In Google Analytics this can be considered as one session, but two channel interactions. Therefore, when you add up the channels individually, it could exceed the total number of sessions.

If you’re using the Multi-Channel Funnels reports in Google Analytics, they would attribute a single session to different channels which might lead to a higher session count when you add up channels manually.

In essence, the difference doesn’t imply an error in counting, but rather it’s a reflection of how Google Analytics attributes sessions to different channels. To get an accurate representation, it’s more beneficial to analyze the performance of each channel on its own, rather than try to combine them for a total. Understanding each channel’s contribution can give you a better impression of which are successfully driving users to your website.