Why do all my NTC thermistor have a PTC curve?
As general troubleshooting guide, if you question the thermistor, test it in isolation. Disconnect the thermistor from the circuit, and use a multimeter in resistance mode to measure its resistance... should show about 100 kohm. Grab the thermistor with your fingers to heat it, and verify that the resistance decreases. Then it's a NTC.

If the problem is not with the thermistor, it's the calculations. Your calculations don't look right to me but I can't say exactly what's wrong. You can't measure Vref as that's already the reference the ADC measures again -- it's like using a ruler to measure the length of the ruler.

As your thermistor is on the high side of the voltage divider and you measure across the lower resistor, you will measure higher voltage when a NTC thermistor heats up. Maybe this is what confuses you.

A warning about your setup; your voltage divider using a 100 kohm thermistor and 100 kohm resistor will together give 50 kohm impedance. Signal impedance is the resistance 'experienced' by the microcontroller ADC, or loosely how sensitive the signal is to disturbances. Arduino Uno uses the ATMega328 microcontroller, which needs lower impedance for ADC to avoid skewed measurements. The datasheet can tell you in detail but I vaguely remember it should be lower than 10 kohm at least. The easiest solution is to add a capacitor between A0 and GND. Any capacitor value 10 nF or higher is ok -- 100 nF and 1 uF are common values.


  The precision, I think, is also affected, some are too 5C others are to 1C.


This may be due to the impedance mentioned above, some error in the calculation or natural variations between thermistors. (All thermistors only promise to follow the specifications within some percentage.)