I'm not entirely familiar with the internals of the ESP32 processor but it seems to have the same kind of internal analog reference as the Arduino Uno: Quite stable but not very accurate. The internal reference is 1.1V +/- 10%. Since your outlier is reading about 5% low that may be explained by a natural variation in analog references. If that is the case you will need to calibrate your calculations for each processor.
On the UNO it's easy because it has an Aref pin which is connected to the internal reference when you select INTERNAL rather than DEFAULT. I don't know how you would measure the reference on an ESP32.