Here's an example of some syncing weirdness that happened to me just the other day. I've been reading "In the Garden of Beasts" by Erik Larson. I was reading primarily on the Touch, connected to Wi-Fi. Picking up my iPad, it would sync to the most recent page that I'd been reading about half the time. About 3/4 of the time, after reading on the iPad, the Touch would recognize that I had read to a new page.
As if that weren't enough, a couple of days later I opened up the iPhone app and opened the book. It was on the first page of the book, even though I was halfway through on the iPad and Touch. I exited the app and opened the book on the iPad... which then opened to the first page of the book! It had synced the very first page of the book as my last page read.