From Arduino Foundations:
The ATmega datasheet also cautions against switching analog pins in close temporal proximity to making A/D readings (analogRead) on other analog pins. This can cause electrical noise and introduce jitter in the analog system. It may be desirable, after manipulating analog pins (in digital mode), to add a short delay before using analogRead() to read other analog pins.
https://docs.arduino.cc/foundations/microcontrollers/analog-input
The Atmel datasheet advised to read twice and keep the 2nd but did give a "settle time" shorter than a full read.
Do you measure each 10R resistor as exact 10.0?
Arduino has no FPU, the floats are 32-bit piles of sand so why not work with integers and small units? It'll run faster and hold precision.
Work in long microvolts and Ohms. You can lose 3 places in a divide and still have precise millivolts.
10R + 100R divider, 5VCC
Vout = (5000000uV * 100R) / (10R + 100R) = 4,545,454uV = 4545mV = 4.545V