nodemcu/esp8266 reads lower than expected on A0

I got a NodeMCU/ESP8266 development board and am trying to utilise the A0 (10-bit ADC) input to read the voltage from a voltage divider on the 3.3V pin. So I have the following:

^{simulate this circuit – Schematic created using CircuitLab}

However when I read A0, it's not the ~512 value that I'd expect for the 1.65V (as measured with a multimeter) that is on A0. It's rather somewhere around 450.

If I swap out the 100K resistors for 1M resistors (which is my target because I have a 1M thermister), the reading is even more off, around 200 where the voltage on A0 reads about 1.55V with a multimeter.

Even with 3.3V on A0, it's only reading 1016-1017 and with nothing on A0 it reads 9 which isn't hugely out, but it's still not hugely accurate either.

Is there something I am not understanding and not taking into account? I'd be interested in any insight.