Wrong message count on IMAP servers

[j.cubizolles@free.fr]

I've set up gnus to read mail from two IMAP servers, but the read message
counts are awfully wrong :

* at free.fr (french provider) gnus announces 301740 unread emails
  instead of the 1732 from the webmail

* at gmail.com gnus announces 862 unread emails instead of 520 from the
  webmail.

This makes commands like / o N completely useless since I need to ask
for N in the hundreds to get a few emails.

Could it be that some old emails are not completely deleted from the
imap server but nevertheless not displayed by the webmail ? 

Is the number of read imap emails correct for some of you ?

Here are the nnimap lines from my .gnus.el

,----
| (setq nnimap-fetch-partial-articles "text/")
| (setq gnus-secondary-select-methods
|       '(
| 	(nnimap "gmail"
|                 (nnimap-address "imap.gmail.com")
| 		(nnimap-authentificator "j.cubizolles")
|                 (nnimap-authinfo-file "~/.authinfo")
|                 (nnimap-server-port 993)
|                 (nnimap-stream ssl))
| 	(nnimap "free"
| 		(nnimap-address "imap.free.fr")
| 		(nnimap-authentificator "j.cubizolles")
| 		(nnimap-authinfo-file "~/.authinfo")
| ;		(nnimap-server-port 143)
| 		)
| ))
| (setq nnimap-split-inbox '("INBOX"))
| 
| (setq nnimap-split-rule 'nnimap-split-fancy)
| (setq nnimap-split-fancy
|    `(|
|         (: gnus-registry-split-fancy-with-parent)
|         (: spam-split 'spam-use-bogofilter)
| 	(any "\\b\\(\\w+\\)@.*bretagne.*" "ml.ups.\\1")
| 	(any ".*ups-\\b\\(\\w+\\)@listes\\.prepas\\.org" "ml.ups.\\1")
| 	("gnus-warning" "duplicat\\(e\\|ion\\) of message" "duplicate")
|     )
| )
`----

I'm using the gnus version shipped with GNU Emacs 24.0.50.1.