Analog thermistor readings are wrong
One possibility that has not been mentioned yet: the issue may be
the calibration of your ADC converter. Your code carries the implicit
assumption that the voltage you measure is

V = Vref × RawADC ÷ 1023


The correct formula is

V = Vref × (RawADC + offset) ÷ scale


where offset is ideally 0 and scale is ideally 1024 (and not 1023,
as you are assuming). Since you are looking at relatively small
discrepancies, the “ideal ADC” assumption may not be a good enough. I
have calibrated 3&nbsp;Arduino ADCs and found offsets between 2.2 and
2.4, and scale factors between 1026.1 and 1026.3. Thus, if you have an
accuracy issue, I recommend you calibrate your own ADC.

As a side note, you could expect slightly better accuracy if the
thermistor and the resistor are closer in resistance. Right now you have
a factor 3, which is not that bad, but you could use a 2.2&nbsp;kΩ
resistor, which is a very standard value.