Additional Information
Without using Offset Compensated Ohms, the error is due to the fact that the relays used in the PXI-2535 and PXI-2536 have additional leakage current that is being applied to your system as shown in the image below.
The expected typical value for leakage current at 25C is: +/-2nA for every closed row/ column connection, +/-1nA for every open column, and +/-1nA for every open row connection. These leakage currents can cause incorrect measurements when using a DMM in the configuration mentioned in
NI PXI-2535 or PXI-2536 FET Switch Doesn't Close. This is caused because of the voltage divider that is created when using a 10Mohm to stabilize the voltage levels on a PXI-2525 / PXI-2526 as shown below.
The expected error that you might receive in your test setup would be system dependent. NI recommends that you use experimental data to calibrate the measurements appropriately. Listed below would be the worst case errors you may be seeing when operating at 25 C.