You also have to have logpage enabled--that's how FFF calculates words_added.
From defaults.ini:
Code:
## include a Update Log page before the story text. If 'true', the
## log will be updated each time the epub is and all the metadata
## fields that have changed since the last update (typically
## dateUpdated,numChapters,numWords at a minimum) will be shown.
## Great for tracking when chapters came out and when the description,
## etc changed.
include_logpage: false
## If set to 'smart', logpage will only be included if the story is
## status:In-Progress or already had a logpage. That way you don't
## end up with Completed stories that have just one logpage entry.
#include_logpage: smart