Hi -
This could happen if you are moving the USB stick between timezones. We have had people complaining about initial sync's having to sync all files because of the time change but not incremental syncs.
Another option you could look into would be to format the USB stick as NTFS - the article below talks about how to do this and the pros and cons. You should read it and see if it would work for your scenario.
http://www.online-tech-tips.com/computer-tips/format-usb-ntfs/Thanks
Deepa
Deepa ( Microsoft Sync Framework)