Stats are updated when logs are deleted. The problem is that Project-GC doesn't get notifications when logs are deleted (or edited, for that matter), so it may take a long time before we detect that. Either it happens the next time the cache is logged or (failing that) the cache will eventually be refreshed anyway but that may take a month if you are unlucky. You can always go to Support -> Self-support and refresh a specific cache (or all your logs) if you are in a hurry.