I am syncing between Kindle iPhone, PC, Mac and K3. Sometimes when i sync it does not take me to the furthest location, but a couple of syncs before. Like my profile hasn't really been updated.
Unless you force a manual sync as soon as you finish on one device, and then force one again immediately when you pick up the new device, then yes... if you do a lot of switching between devices (rather quickly) you may just catch an ebook that is temporarily out of sync with your furthest read page. I don't think it was ever intended to be an immediate, real-time sort of thing.