At the moment the CSV Documentation does not explain field mappings very well. Especially with the 'Import as Blank' Option.

From a support email:
"When importing the CSV file containing our defect data, I populated the
resolution field incorrectly. When I mapped our string resolutions (fixed,
cancelled, etc) with the pull down list in the importer, I left the items
that I wanted to keep open as "No Mapping" instead of "Enter as Blank".
This caused problems in the database because the items that I had mapped
were entered as their numerical ID for the status, and the items I had
selected as "No Mapping" had their strings in the table instead of the
blank I had wanted. On the list of issues, these issues with incorrect
resolutions were listed as "UNRESOLVED", however they did not appear in any
of the "Open Issue" lists. When I went back and reimported the data with
the "Enter as Blank" everything worked fine."