2004 Nissan Xterra ODO shows more miles than the computer

Hello. We got this Xterra. What do you think would cause the odometer to show More miles than the on board computer reads when we had our state inspection? The story actually gets more interesting because the registration and milage disclosure shows a milage which was 900 miles less than actual and they told us to drive around until the odometer catches up with the paperwork. So when we did finally get the inspection the on board computer showed 2000 less than actual. We got this from a new car dealer as a used. What the heck do you think is going on? Thanks, Joe