Modules are assigned to specific pages using a list of menu links that is visible when you edit the module in the module manager (on the right of the page). This list does NOT include menu links of the type "URL."

This is an anomoly from the history of the development of Joomla! carried over from Mambo. Although you can use the url type for any link, including internal joomla links, the original idea was that these would be used for links to external sites which would not display joomla! modules.

Some Joomla! component extensions use url links (rather than component links with parameters) as a simple way to access specific views of the components.

In Joomla! 1.0.12 and later this issue was partially addressed by allowing modules to be assigned to URL links that did not include ItemId. Therefore, one way to address this issue is to make sure that your site is running the current version of Joomla! and that url links that are internal to your site do not include ItemId values.