I'm surprised this hasn't come up much, but some styles require the total number of pages when citing reports. One example is the National Library of Medicine, which can be seen here:

http://www.ncbi.nlm.nih.gov/books/NBK7280/#A38186

However, the only page field in Zotero for reports is a page range, unlike in books where it has a field for "# of pages".

I did find an old reference to this issue from 2012 (see https://forums.zotero.org/discussion/23490/style-request-weed-science-weed-technology/ for the discussion and another example), which states that it would "fix itself", but it may have slipped through the cracks...