What would cause an unexpected massive transfer

[Harry Putnam]

I've rsynced two directory structures back and forth a few times.

Its a desktop to external (portable) drive.  And then back once mobile
work has changed files and field work is done.

On this last go around I expected a few files to be transferred maybe
15-20 but instead I'm seeing several thousand.

No way most of those were changed.  At least not data wise.

It doesn't appear that I've missed my aim and built a new structure by
putting a wrong path in the command so I'm thinking it has to be some
superficial dating or other non data change that is provoking rsync.

The command was:

  rsync -avvzP --stats  /mnt/harvey-g/ /mnt/harvey-ext/Harvey_G/ 

I'm watching as I write and see old files that haven't changed in
literally years being transferred.

The file systems in volved are (xp)NTFS on one end and Fat32 on the
external drive.

The two partitions are mounted on a linux (gentoo) server where the
rsync command is run from.

I'm transferring a big pile of image data that doesn't need to be
moved. 

Any ideas why this would happen after several normal rsyncs on these
directories both directions.