Answer: The IP330 is a highly accurate instrument and you should be able to get the rated accuracy from the unit.
First, The IP330 uses an extremely high impedance instrument amplifier. This has the desired effect of improving the overall accuracy. It has one undesired effect: unused channels are briefly scanned by the multiplexer and drive the amplifier into saturation. Recovery from saturation is slow and greatly affects the readings of the desired channels. The solution is simple. Tie off all unused inputs to analog common. This is good practice anyway because floating inputs are generally undesirable for noise immunity.
Second, if you are using Differential Inputs, you must reference the minus differential signal to Analog Common. Remember differential does not mean isolated. If you leave them floating, the inputs could easily drift above the allowable input range.