Odd behavior of rsync on Mac OS X

Today, I was trying to type an rsync command that would copy all my iPhotos from one computer to another. Since the final directory in the path contains a space character ("iPhoto Library"), I included it in single quotes, like this (username and computer are not the actual values I used :-)

rsync -avz username@computer.local:/Users/Shared/Pictures/'iPhoto Library/' /Users/Shared/Pictures/'iPhoto Library/'

After pressing Enter and entering my password, rsync gave the error:

receiving file list ... rsync: link_stat "/Users/Shared/Pictures/iPhoto" failed: No such file or directory (2)

Clearly, the single quotes weren't doing the job. So I replaced single with double-quotes. Still no luck. After googling space chars in directory names, and learning about the backslash, I tried 'iPhoto\ Library/'. This kinda worked: Rsync found the right source directory, but on the destination machine it created a directory named iPhoto\ Library. But at least the results suggested a solution. I changed the destination directory to 'iPhoto Library/', so the entire command looked assymetrically like this:

rsync -avz username@computer.local:/Users/Shared/Pictures/'iPhoto\ Library/' /Users/Shared/Pictures/'iPhoto Library/'

Eureka! It works. My only problem is I don't know why the specification of the source directory needs both single-quotes and a backslash to represent a space character, but the destination directory needs only single-quotes and a space?

-Xot