When you manually install a font or copy a font into the Fonts folder using the shell methods, explorer.exe will call this AddFontResource API too (and again explorer.exe runs in each user's session).
A specific Windows API function (AddFontResource) needs to be called to actually make a font available to a user' session, and this happens during logon as winlogon.exe loops through each value in the reg key that has already been mentioned and for each one calls the AddFontResource API to install that font for the user (winlogon.exe runs in the user's session so the fonts it installs are available for that user when the logon has completed). However, every time a user logs on every font that has been installed on the system gets added to the list of fonts that are available for that logon session.
When a font is installed it is only made available to that user's session that it was installed from. Just to explain why you need to log back on after installing a font from a computer startup script or GPO Preference (or anything not running as the currently logged on user) :
