What does your deep dive entail? Do you have any filters in the Smart List tab? Does your email send to the same audience of people more than once? There's a few things that can influence what your report is actually calculating and that could differ from your expectations. It's hard to suggest what it could be without knowing how your report is actually set up versus knowing how you came up with the 18% number (open rate?). Support can dig into it if you open a ticket with a link to the report and your separately calculated data points.
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