documenting system_cron impact on page cache?

Apologies if this is filed in the wrong place, but I have been unable to find documentation anywhere else.

I am confused about the behavior of the page cache in D6, which seems different from D5. I think that the inclusion of cache_clear_all(NULL, $table); in the system_cron() function significantly alters the way page cache works.

I am testing with minimum cache lifetime set to "none," though I believe other configurations would be affected as well (in slightly different ways).

In D5, which lacked the system_cron() function, expirable entries in page cache would only be deleted after specific events occurred. For example, creating or editing a node would cause expirable entries to be deleted. (Presumably because these actions were likely to cause the entries in page cache to become stale.) Visiting cron.php would not delete all expirable entries unless such an event had not taken place.

In D6, all expirable entries are deleted every single time cron.php is hit. This is true even if no changes have been made to the site (no nodes created or edited, etc).

Is the above description correct? If so, should/could this change in behavior be documented somewhere?

In my case (and perhaps others), it is desirable to have the entire page cache persist until the occurrence of an event that changes the site's data (creation / edit / deletion of a node, etc). For my purposes, hitting cron.php should not automatically wipe the page cache of all expirable entries. Is it possible to accomplish this in D6?

Thank you for your help!