Privacy fail? Spotlight shows the Safari pages read by other user accounts
If it is a bug rather than a setting, it's never going to get fixed in a 2-year-old OS, only in some future update.
It still does it in Yosemite, btw, just tested.

You could add all other users' home folders to the Privacy tab in Spotlight prefs - but this, of course, would require the complicity of each user.