I got the explanation from one OpenOffice expert:
"hand" modification of the format of a paragraph triggers the creation of a so-called "automatic" style, whose description is kept in
content.xml, while so-called "normal" styles descriptions are kept in
styles.xml. These normal styles are created by using F11 stylist window and are the normal way to proceed for a big text.
So, if I hand modify twenty paragraphs, it will create twenty similar styles (only differing with identification number). The same applies for hand formatting of characters.
So, I am obviously the guilty one and the difference between automatic and normal styles explains the sudden appearance of so many similar styles.
He said it in French.
La modification "à la main" du formatage d'un paragraphe provoque la création d'un style automatique dont la description est stockée dans content.xml alors que les styles "normaux" sont stockés dans styles.xml. Même si toutes les modifications sont les mêmes, il y aura un style automatique pour chaque paragraphe modifié. Même chose pour les modifications de mise en forme de caractère.