D6 Cron mystery

I have a module that hooks nodeapi and if an update has been performed on the correct content type, I create an XML file and transmit it.

The problem is that the XML files were being created en masse, where normally an editor edits a single piece of content. The culprit has tentatively been identified as a cron run that updates each node with Facebook 'like' statistics, which performs a node save. Interestingly, some settings in the file end up being different...settings that should in no way be affected by what triggers the update, given that the node data used is not different in either case.

I put the following line in the hook function:

if (variable_get('cron_semaphore', FALSE) || user_is_anonymous()) return;

I would think that a cron initiated trigger would fail for both conditions. It does not fail at all. I'm stuck!