EKMallon:
Thanks for clarifying that for me Mark. I had forgotten about the pins 'floating' when in input mode.
In your case, not even floating.
You have your voltage divider between the pin and GND. So if you set the pin to INPUT, LOW the pin will be pulled low by the about 20k resistance in the divider (assuming 10k NTC & 10k pull-up resistor).
Setting the pin to INPUT_PULLUP you enable the internal resistor, which tries to pull up the pin, and you'll end up at roughly 2V (30k internal and 20k external) on the pin.