Translation Word Count: Why Do Word Counts Vary From Agency to Agency?
http://www.wintranslation.com/articles/art05_0008_wordcount.htm

Bologna, Sandra
WTB Language Group
2004

Abstract:

From generating a word count using the Word Count tool in MS Word, or using a counting tool such as PractiCount and Invoice, to using translation tools such as Trados or Wordfast, it's hard to produce similar results when each of these tools have their own way of counting.