RANK behaving strangely on Oracle 9.2

Hi, 

I'm facing a strange behaviour of the RANK() function on an old Oracle 9.2 database.

Here is the SQL Query : 

SELECT 

    RANK() OVER (order by mco_rum.date_sortie NULLS FIRST) AS NUM,

    COUNT(*) OVER (order by mco_rum.date_sortie NULLS FIRST) CTN,

    mco_rum.identifiant AS idRUM,

    TO_CHAR(mco_rum.date_entree,'DD/MM/YYYY'),

    TO_CHAR(mco_rum.date_sortie,'DD/MM/YYYY'),

    mco_rum.codeunitesoin,

    mco_rum.verrou,

    mco_mode_entree.libelle AS modeEntree,

    mco_mode_sortie.libelle AS modeSortie,

    mco_rss.identifiant    AS idRSS,

    mco_rss.numero_hospitalisation,

    mco_resultat.cmd,

    mco_resultat.code_retour,