A few
chapters back?

That definitely should not be occurring. Imprecision has in some cases been observed in the range of several
page flips, at most (discussed
here), but location sync should definitely put you in the same
chapter, because that's how it's designed to work.
I suggest that you do not sync your location by manually pressing the sync button (because that can cause confusion: see
here).
Please try syncing your position on the 1st device simply by backgrounding Marvin (by returning to your device's home screen). Then, pick up your 2nd device, press the sync button and perform the sync. I believe you should get a much better result. Please allow a couple of minutes to pass (at least initially for the first test, to make sure) before picking up the 2nd device.