How to mirror one folder to another using rsync 3.x?
If you add the itemised "-i" or "--itemize-changes" option to your command rsync will tell you why it is transferring the files.

For each file you'll see a strings that looks like ">f..t......". The format of the string is "YXcstpoguax". Y is the direction of the transfer. X is the type of object being transferred (for example 'f'ile, 'd'irectory). The rest of the string documents the changes.


c for checksum
s for size
t for modification time
p for permissions
o for owner
g for group
u is currently reserved for future use
a for ACL permissions
x for extended attributes


So in my example above it transferred the file because the modification time was different.

This should help you identify why the files are still being transferred.