The Short Stats "max online" is set by the "Whos Online" pal box which is checked at every index page load. If your not displaying the "whos online" information on the index page then this information will not be tracked. This Short Stats information is indeed cached though but as far as the max online data goes it is updated in the cache field at every page load.
The only way this information can be stored is if someone loads the index page so that the "whos online" pal box can count the users and store them. If nobody enters your site through the index page this information will not be stored.
I'm not sure what to say about this though. I have looked things over and can't see where it would be going wrong.
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