Hi,
What you see is an expected behaviour when using the SCXI-1303 isothermal terminal block. For each input channel, the terminal block has a pullup resistor connected between CH+ and +5 V and a bias resistor connected between CH– and chassis ground. These resistors help you detect open thermocouples by saturating the module amplifier output.
I recommend you to read the user manual of the SCXI-1303 for more details: http://www.ni.com/pdf/manuals/321923b.pdf
Hope that helps.
Regards,
Luca