NTC3950 is not really a thermistor type. It is just a resistance/temperature curve specified and you can only hope it follows really that curve. This is for 25°C and the farther away you go the more they tend to deviate from the formula.
Anyway, at room temperature it should not be that much off. Result is influenced by 2 things.
1. Thermistor resistance. At 25°C it should have 100kOhm, you should be able to verify that,
2. On board is a 4,7K resistor that reduces current through thermistor and influences voltage measured. Make sure it is 4.7k of the build in curves do not work. If not you need your own curve with different R1.
Make sure you did not use a thermocouple input, that would not work as well.
