Hello,
in at least the 20060915 frwiki dump, the is_redirect column (in the page table) is 0 for a wide variety of articles where it should be 1 (first one of them is page_id=204, then 758, 917, ...).
These articles all have in common that they had a life before being declared as redirects, and when they were, the is_redirect field was apparently not updated to reflect the new state.
Sorry if it is a known bug (is there a public BTS for mediawiki?).
Regards, Colin