I think the issue is that you're sorting by the id.
The fulltext sorts by the match score it calculates, showing stronger matches first. When you apply ORDER BY id DESC, you loose this sort-by-match ordering.
You can see the actual score in your result set if you want by:
AGAINST ('your term' IN BOOLEAN MODE) AS score
WHERE MATCH (art,tit,tem,cred,info)
AGAINST ('your term' IN BOOLEAN MODE)
ORDER BY id DESC
BTW: Use prepared statements for the 'your term' portion.
If your search string has spaces but each term matter, you need to treat them as separate pieces. So if it's important to have BOTH "Mercedes" AND "Benz":
Don't: AGAINST ('Mercedes Benz' IN BOOLEAN MODE) <--- This means either Mercedes or Benz
Do: AGAINST ('+Mercedes +Benz' IN BOOLEAN MODE)
If you want to have anything that must have the first term, but optionally the second term (ranking higher when both found) do: