It may be because of the high impedance (the big resistors). There is a 4.9k resistor and a .15 uf capacitor on the lines going into the AD7298. That means there is a timing requirement for the capacitor and with a fixed conversion time, there are charging problems. I think that if you used a unity gain op amp to condition the input you would see the correct values when you use the larger resistors (This is a nice write up http://ww1.microchip.com/downloads/en/AppNotes/00682D.pdf).
Then again, I am not an engineer and can't even get a running timer working inside of a sketch, so it may be advisable to wait for someone else to answer
...and, in the case that I am on at least the right track, could the maximum impedance be calculated so that I could understand?