If I set Channel 1 of U2300A/U2500A with voltage range of 5V, I get a reading of 4.99968 when nothing is connected to the DAQ.

If you don't have anything connected to the DAQ and you do an acquisition out of it, the result that you'll get will depend on the positive and negative power line that you've set (voltage range).
If you set in bipolar mode a voltage range of 5V, you will either get 4.9xxx or -4.9xxx readings.

If you set in unipolar mode a voltage range of 2.5V, you will either get 2.4xxx or 0.xxxx readings.

This is the behaviour of the internal design of the DAQ.