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did the records/transactions you're searching change between running the search and exporting it (exporting will get fresh results, can check this by running and seeing if results are 980/different)
Are there "available filters" involved? Particularly if you have a text filter that is defaulted to a value by the search criteria, then you delete the filter text then export, sometimes the default filter text (again, set by the search criteria) will get applied.
Those are two reasons that spring to mind

not in my experience. Sometimes to limit the initial results (so users can load the search faster) I'll put the record name as an available filter, with a search criteria of "placeholder". that way the end user can load the search (no results yet) and replace the text "placeholder" with the record name they are looking for.
If you just delete the filter text you'll get all results, however, if you then export the search NS will reapply the "placeholder" filter and you'll get an empty CSV. Not sure why this happens.

Is this similar to what you're experiencing?

Is it a summary search? I think sometimes the line count listed for summary searches isn't actually what's exported