Answer: For some obscure reason, Microsoft prefers to display the same attribute as "N" in Windows Explorer and as "I" in the command line, at least in Windows Vista (not tested on other versions of Windows).
I have performed tests using the attrib.exe command and as users Hennes and Mr. Alien stated, inded the "N" attribute displayed on the attributes column of Windows Explorer means "not indexed", while the same attribute is displayed as "+I" on the command line via attrib.
The folders containing the "N" attribute lacked the checkmark on "Index this folder for faster searching", whereas the folders without this attribute had the checkmark.
Incidentally, I found out Windows won't let me change the attributes of either hidden or system files/folders. This way, if I want a file/folder to be both +H and +S, I have to set both attributes at the same time.