Hi Ana-Maria Matefi (Ana-MariaMatefi) ,

Thanks for your answer. Unfortunately it doesn't answer my question :(
That article proposes a way to estimate the "target word count".

My problem here, is that two different versions of Trados Studio (2014 and 2015) are not giving the same amount of source new words, but the same fuzzy matches.
It seems to be an bug but maybe is there a logic behind this ?

Wish you a great day too,

Best regards,

Vincent

Probably THIS is the blog post Ana-Maria actually wanted to refer to:
multifarious.filkin.com/.../

You should be able to get the idea that "what is considered a word" differs not only between various tools, but also between various versions of the same tool... and that it's most probably what you are experiencing.
More detailed analysis would require looking in way more details at particular cases, i.e. deeply analyzing your particular files.